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A printing apparatus performs printing on a print medium. The printing
apparatus includes: a dot data generator that performs a halftone process
on image data, wherein the print image is formed by mutually combining
print pixels belonging to each of a plurality of pixel position groups
for which a physical difference is assumed at a formation of dots by the
print image generator, in a common print area, and the halftone process
is configured to determine the status of dot formation on each of the
print pixels on an assumption of the physical difference.

Inventors:

Kakutani; Toshiaki; (Nagano-ken, JP)

Serial No.:

035361

Series Code:

13

Filed:

February 25, 2011

Current U.S. Class:

358/3.06

Class at Publication:

358/3.06

International Class:

H04N 1/405 20060101 H04N001/405

Foreign Application Data

Date

Code

Application Number

Feb 9, 2005

JP

2005-32771

Jun 10, 2005

JP

2005-171290

Jul 21, 2005

JP

2005-210792

Claims

1. A printing apparatus that performs printing on a print medium,
comprising: a dot data generator that performs a halftone process on
image data representing a tone value of each of pixels constituting an
original image to determine a status of dot formation on each of print
pixels of the print image to be formed on the print medium, for
generating dot data representing the determined status of dot formation,
and a print image generator that forms a dot on each of the print pixels
for generating a print image according to the dot data, wherein the print
image is formed by mutually combining print pixels belonging to each of a
plurality of pixel position groups for which a physical difference is
assumed at a formation of dots by the print image generator, in a common
print area, and the halftone process is configured to determine the
status of dot formation on each of the print pixels on an assumption of
the physical difference.

2. The printing apparatus according to claim 1, wherein the physical
differences include a displacement of timing of dot formation for each of
the plurality of pixel position groups.

3. The printing apparatus according to claim 2, wherein the halftone
process is configured such that each of spatial frequency distributions
of the dot patterns formed on the print pixels belonging to each of the
plurality of pixel position groups and spatial frequency distributions of
the print image have a mutually positive correlation coefficient of at
least 0.7.

4. The printing apparatus according to claim 1, wherein the physical
differences include a relative shift of dot position for each of the
plurality of pixel position groups.

5. The printing apparatus according to claim 4, wherein the halftone
process is configured such that each of spatial frequency distributions
of the dot patterns formed on the print pixels belonging to each of the
plurality of pixel position groups and spatial frequency distributions of
the print image have a mutually positive correlation coefficient of at
least 0.7.

6. The printing apparatus according to claim 1, wherein the halftone
process is further configured to provide each of dot patterns with a
predetermined characteristics, the each of dot patterns including dot
patterns formed on the print pixels belonging to each of the plurality of
pixel position groups.

7. The printing apparatus according to claim 4, wherein the halftone
process is further configured to provide a hypothetical print image on an
assumption of none of the relative shift of dot position with a
predetermined characteristics.

8. The printing apparatus according to claim 4, wherein the halftone
process is further configured to provide both of a plurality of
hypothetical print images with a predetermined characteristics, the
plurality of hypothetical print images including a first hypothetical
print image on an assumption of the relative shift of dot position and a
second hypothetical print image on an assumption of none of the relative
shift of dot position.

9. The printing apparatus according to claim 1, wherein the specified
characteristics is either one of blue noise characteristics and green
noise characteristics.

10. The printing apparatus according to claim 1, wherein the print image
generator has a printing head and generates a print image by forming dots
on each of the print pixels during forward scan and backward scan of the
printing head, while performing a main scan of the printing head, and the
plurality of pixel position groups includes a first pixel position group
for which dots are formed during the forward scan of the printing head
and a second pixel position group for which dots are formed during the
backward scan of the printing head.

11. The printing apparatus according to claim 1, wherein the print image
generator has a printing head and generates a print image by forming dots
on each of the print pixels while repeating a main scan cycle of the
printing head N times (N is an integer of 2 or more), according to the
dot data, and the plurality of pixel position groups includes a plurality
of pixel position groups divided according to a remainder from a
numerical value representing an order of a sub-scan direction of the main
scan line divided by the aforementioned N.

12. The printing apparatus according to claim 1, wherein the print image
generator has a plurality of printing head and generates a print image by
forming dots on each of the print pixels according to the dot data, while
performing a main scan of the plurality of printing heads, and the
plurality of pixel position groups includes a plurality of pixel position
groups for which each of the plurality of printing heads in charge of the
dot formation of each of the plurality of pixel position groups.

13. The printing apparatus according to claim 1, wherein the print image
generator has a plurality of printing head and generates a print image by
forming dots on each of the print pixels according to the dot data, while
performing a sub-scan of the print medium, and the plurality of pixel
position groups includes a plurality of pixel position groups for which
each of the plurality of printing heads in charge of the dot formation of
each of the plurality of pixel position groups.

14. The printing apparatus according to claim 10, wherein the halftone
process is configured such that each of spatial frequency distributions
of the dot patterns formed on the print pixels belonging to each of the
plurality of pixel position groups and spatial frequency distributions of
the print image have a mutually positive correlation coefficient.

15. The printing apparatus according to claim 10, wherein the halftone
process is configured such that each of spatial frequency distributions
of the dot patterns formed on the print pixels belonging to each of the
plurality of pixel position groups and spatial frequency distributions of
the print image have a mutually positive correlation coefficient of at
least 0.7.

16. The printing apparatus according to claim 10, wherein the halftone
process is configured such that any first correlation coefficients are
higher than any of second correlation coefficients at least for tone
levels with relatively low dot density, wherein the first correlation
coefficients are coefficients between each of spatial frequency
distributions of the dot patterns formed on the print pixels belonging to
each of the plurality of pixel position groups and spatial frequency
distribution of the print image, and the second correlation coefficients
are correlation coefficients between each of spatial frequency
distributions of dot patterns formed on print pixels belonging to each of
any of other plurality of the pixel position groups that form print
images by mutually combining in a common print area and the spatial
frequency distribution of the print image.

17. The printing apparatus according to claim 10, wherein the halftone
process is configured such that a RMS granularity of dot patterns formed
on the print pixels belonging to each of the plurality of pixel position
groups is lower than the RMS granularity of dot pattern formed on the
print pixels belonging to each of any of other of the plurality of pixel
position groups that form print images by mutually combining in a common
print area, at least for tone levels with relatively low dot density.

18. A printing method of printing on a print medium, comprising:
performing a halftone process on image data representing a tone value of
each of pixels constituting an original image to determine a status of
dot formation on each of print pixels of the print image to be formed on
the print medium, for generating dot data representing the determined
status of dot formation, and forming a dot on each of the print pixels
for generating a print image according to the dot data, wherein the print
image is formed by mutually combining a plurality of pixel position
groups for which a physical difference is assumed at a formation of dots
by the print image generator, in a common print area, and the halftone
process is configured to determine the status of dot formation on each of
the print pixels on an assumption of the physical difference.

19. The printing method according to claim 18, wherein the physical
differences include a displacement of timing of dot formation for each of
the plurality of pixel position groups, and the halftone process is
configured such that each of spatial frequency distributions of the dot
patterns formed on the print pixels belonging to each of the plurality of
pixel position groups and spatial frequency distributions of the print
image have a mutually positive correlation coefficient of at least 0.7.

20. The printing method according to claim 18, wherein the physical
differences include a relative shift of dot position for each of the
plurality of pixel position groups, and the halftone process is
configured such that each of spatial frequency distributions of the dot
patterns formed on the print pixels belonging to each of the plurality of
pixel position groups and spatial frequency distributions of the print
image have a mutually positive correlation coefficient of at least 0.7.

21. A computer program product for causing a computer to generate print
data to be supplied to a print image generator for generating a print
image by forming dots on a print medium, the computer program product
comprising: a non-transitory computer readable medium; and a computer
program stored on the non-transitory computer readable medium, the
computer program comprising a program for causing the computer to perform
a halftone process on image data representing a tone value of each of
pixels constituting an original image to determine a status of dot
formation on each of print pixels of the print image to be formed on the
print medium, for generating dot data representing the determined status
of dot formation, wherein the print image is formed by mutually combining
print pixels belonging to each of a plurality of pixel position groups
for which a physical difference is assumed at a formation of dots by the
print image generator, in a common print area, and the halftone process
is configured to determine the status of dot formation on each of the
print pixels on an assumption of the physical difference.

22. The computer program product according to claim 21, wherein the
physical differences include a displacement of timing of dot formation
for each of the plurality of pixel position groups, and the halftone
process is configured such that each of spatial frequency distributions
of the dot patterns formed on the print pixels belonging to each of the
plurality of pixel position groups and spatial frequency distributions of
the print image have a mutually positive correlation coefficient of at
least 0.7.

23. The computer program product according to claim 21, wherein the
physical differences include a relative shift of dot position for each of
the plurality of pixel position groups, and the halftone process is
configured such that each of spatial frequency distributions of the dot
patterns formed on the print pixels belonging to each of the plurality of
pixel position groups and spatial frequency distributions of the print
image have a mutually positive correlation coefficient of at least 0.7.

24. A printing apparatus that performs printing on a print medium,
comprising: a dot data generator that performs a halftone process on
image data representing a tone value of each of pixels constituting an
original image to determine a status of dot formation on each of print
pixels of the print image to be formed on the print medium, for
generating dot data representing the determined status of dot formation,
and a print image generator equipped with a printing head configured to
generate a print image by forming dots on each of the print pixels during
forward scan and backward scan of the printing head, while performing a
main scan of the printing head, wherein the print image is formed by
mutually combining print pixels belonging to each of a plurality of pixel
position groups including a first pixel position group for which dots are
formed during the forward scan of the printing head and a second pixel
position group for which dots are formed during the backward scan of the
printing head, and the halftone process is configured such that any of
mutual correlation coefficients between a plurality of spatial frequency
distributions consisting of spatial frequency distributions of dot
patterns formed on the print pixels belonging to the first pixel position
group, spatial frequency distributions of dot patterns formed on the
print pixels belonging to the second pixel position group, and spatial
frequency distributions of the print image is at least 0.7, at least for
the tone level with relatively low dot density.

25. A printing method of printing on a print medium, comprising:
performing a halftone process on image data representing a tone value of
each of pixels constituting an original image to determine a status of
dot formation on each of print pixels of the print image to be formed on
the print medium, for generating dot data representing the determined
status of dot formation, and forming dots on each of the print pixels
during forward scan and backward scan of a printing head, while
performing a main scan of the printing head, for generating a print image
according to the dot data, wherein the print image is formed by mutually
combining print pixels belonging to each of a plurality of pixel position
groups including a first pixel position group for which dots are formed
during the forward scan of the printing head and a second pixel position
group for which dots are formed during the backward scan of the printing
head, and the halftone process is configured such that any of mutual
correlation coefficients between a plurality of spatial frequency
distributions consisting of spatial frequency distributions of dot
patterns formed on the print pixels belonging to the first pixel position
group, spatial frequency distributions of dot patterns formed on the
print pixels belonging to the second pixel position group, and spatial
frequency distributions of the print image is at least 0.7, at least for
the tone level with relatively low dot density.

Description

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation of U.S. patent application Ser.
No. 11/350,374, filed on Feb. 7, 2006. The disclosure of this prior
application is hereby incorporated by reference in its entirety for all
purposes.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to technology for printing an image by
forming dots on a print medium.

[0004] 2. Description of the Related Art

[0005] As an output device for images created by a computer, images shot
using a digital camera, or the like, printing apparatus that print images
by forming dots on a print medium are widely used. These printing
apparatus print images by forming dots on a print medium by driving the
heads at a suitable timing while running the dot forming head back and
forth over the print medium. Also, among the printing apparatus, there
are items that form dots only during the forward scan of the dot forming
head, but if dots are formed by driving the head during the backward scan
in addition to the forward scan, it is possible to print images rapidly.
In this way, a method of printing by forming dots during forward scan and
backward scan is called bidirectional printing.

[0006] With a printing apparatus that performs bidirectional printing,
when forming during forward scan and when forming during backward scan,
it is necessary to make adjustments in advance of the dot forming timing
so as not to have displacement occur for the dot formation positions.
This is due to the following kind of reason. For example, if forming dots
only during forward scan of the head, one reference position near the end
part of the back and forth movement is set, and it is possible to have
the dot formation start when the head passes through the reference
position (or at a specified timing after passing the reference position).
To print one image, it is necessary to have the dot forming head go back
and forth a plurality of times, but if the dot formation starts from the
same position each time during the forward scan, there is no displacement
of the dot position even when forming with dividing of the back and forth
movement into a plurality of times.

[0007] In comparison to this, when forming dots during the backward scan
as well, so that the formed dots start being formed from exactly the end
position of the dot line formed during forward scan, it is necessary to
suitably adjust the timing of starting formation of dots during the
backward scan each time. Of course, so that dots are formed from exactly
the end of the dot row formed during forward scan, even if the timing of
starting dot formation during the backward scan is adjusted, if there was
a tiny difference in the head movement speed during the forward scan and
the backward scan, there will be position misalignment between the dots
formed during forward scan and the dots formed during backward scan.
Because of this, when performing bidirectional printing, the demand for
precision for the mechanism that moves the head back and forth becomes
strict. Then, when sufficient precision cannot be secured, it becomes
necessary to adjust the timing of starting dot formation during backward
scan so that displacements of the dot positions do not show up easily.
From this kind of reason, for the printing apparatus that perform
bidirectional printing, there have been proposed various methods for the
adjustment method, with incorporation of an exclusive adjustment
mechanism for adjusting the relative timing of forming dots during
forward scan and backward scan, and adjustment programs (e.g. Unexamined
Patent No. 7-81190, Unexamined Patent No. 10-329381, and the like).

[0008] However, high precision is demanded for this kind of adjustment, so
there is of course the problem that the adjustment mechanism and
adjustment program become complex and large. Also, to perform
bidirectional printing, because there is demand for high precision for
the mechanism for moving the dot forming head, there is the problem that
the head movement mechanism also tends to become complex and large.
Because of this, even when the dot formation position is slightly
displaced, by suppressing to a minimum the effect on image quality, there
is demand to develop technology that will make it possible to try to
simplify the dot formation position adjustment mechanism and the
adjustment program, as well as the head movement mechanism. Furthermore,
this kind of problem is caused not just by the displacement in the main
scan direction for bidirectional printing, but also due to, for example,
shifts in the dot formation position due to physical reasons including
mechanical errors such as Sub-scan direction displacement or time errors
such as displacement of the ink spray timing. Furthermore, it occurs not
just due to dot formation position misalignment, but also to the
displacement itself of the timing for forming the dots.

SUMMARY OF THE INVENTION

[0009] This invention was created to address the problems described above
of the prior art, and its purpose is to provide technology making it
possible to suppress to a minimum the effect on image quality due to a
physical difference including a displacement of the dot formation
position.

[0010] In order to attain the above and the other objects of the present
invention, there is provided a printing apparatus that performs printing
on a print medium. The printing apparatus comprises: a dot data generator
that performs a halftone process on image data representing a tone value
of each of the pixels constituting an original image to determine a
status of dot formation on each of the print pixels of the print image to
be formed on the print medium, for generating dot data representing the
determined status of dot formation, and a print image generator that
forms a dot on each of the print pixels for generating a print image
according to the dot data. The print image is formed by mutually
combining print pixels belonging to each of a plurality of pixel position
groups for which a physical difference is assumed at a formation of dots
by the print image generator, in a common print area. The halftone
process is configured to determine the status of dot formation on each of
the print pixels on an assumption of the physical difference.

[0011] According to the printing apparatus of this invention, for print
pixels belonging to each of the plurality of pixel position groups for
which physical differences are assumed, a halftone process is constituted
such that the dot formation status on each of the print pixels for which
this physical differences is assumed is decided, so degradation of image
quality due to this kind of physical difference, such as a shift in the
dot formation position or the occurrence of low frequency noise due to
displacement of the dot formation timing, for example, can be suppressed.

[0012] The image quality degradation mechanism due to the organic
relationship between this kind of physical difference and halftone
processing is an insight first found by this inventor. Specifically,
conventional halftone processing was constituted with a focus on the
spatial frequency distribution of the print image, so, for example, if
the relative positions of a plurality of pixel position groups mutually
combined in a shared printing area shift as a single body due to a
physical error of the printing apparatus, the relative positional
relationship collapses, and there is excessive degradation of the image
quality, which was first made clear this time.

[0013] Furthermore, the inventors discovered the following phenomenon.
Specifically, when there is a low frequency dense state for the dots
formed in a plurality of pixel position groups, when there is
displacement of the dot formation timing, and overlapping with this the
ink drops are sprayed, at positions where dot density is high, states
occur such as agglomerations of ink drops, excessive sheen, or a bronzing
phenomenon, and differences in the image occurs between those and
positions at which the dot density is low. This image difference causes
the problem of being easily recognized as image unevenness by the human
visual sense.

[0014] For the printing apparatus noted above, the physical differences
can include displacement of the timing of dot formation for each of the
plurality of pixel position groups, or, the physical differences can
include a shift in the relative position of the dots for each of the
plurality of pixel position groups.

[0015] In this way, physical differences have a broad meaning, of not only
errors in the mechanism of the printing apparatus of printing head
position measurement errors or Sub-scan feed volume measurement errors,
but also, for example, being the cause of main scan direction errors due
to printing paper uplift and ink spray timing (time error) displacement
or sequence.

[0016] Based on this kind of new finding, according to the invention of
this application, for example with the various constitutions like those
shown below, it is possible to suppress the degradation of image quality
due to this kind of physical difference.

[0017] With the printing apparatus noted above, the halftone process can
also be constituted such that any of the dot patterns formed on the print
pixels belonging to each of the plurality of pixel position groups has
specified characteristics.

[0018] In this way, if the dots formed on the print pixels belonging to
each of the plurality of pixel position groups is made to have specified
characteristics, in contrast to the conventional halftone processing that
depends on the relative positional relationship of the plurality of pixel
position groups, it is possible to constitute this as a halftone process
with a high robustness to physical differences.

[0019] Note that the specified characteristics can be decided based on the
granularity index (specifically, the index representing how easy it is
for the dots to stand out), or to have them decided as described later
based on the correlation coefficient of the spatial frequency
distribution. Also, the specified characteristics do not absolutely have
to be provided across all the tones reproduced by this halftone process,
but can also be provided for part of the tones. Here, "part of the tones"
is preferably the tones with a relatively low dot density. This is
because tones with a relatively low dot density make it easier for the
dots to stand out.

[0020] With the printing apparatus noted above, the halftone process can
also be further constituted so that the print images which are assumed
not to include a shift have the specified characteristics. By doing this,
it is possible to further increase the robustness to shifts.

[0021] With the printing apparatus noted above, the halftone process can
also be constituted so that both the print images when it is assumed they
do not include the shift and the printing images when it is assumed they
do include the shift have the specified characteristics.

[0022] By doing this, it is possible to exhibit a marked effect when it is
possible to forecast a shift format.

[0023] With the printing apparatus noted above, the specified
characteristics can be either one of blue noise characteristics or green
noise characteristics.

[0024] With the printing apparatus noted above, it is possible to have it
so that the print image generating unit has a printing head, and while
performing the main scan of the printing head, generates a print image by
forming dots on each of the print pixels according to dot data both
during forward scan and backward scan of the printing head, and the
plurality of pixel position groups includes a first pixel position group
for which dots are formed during the forward scan of the printing head
and a second pixel position group for which dots are formed during the
backward scan of the printing head.

[0025] By doing this, it is possible to constitute the halftone process to
have a high level of robustness to displacement in the main scan
direction for bidirectional printing.

[0026] With the printing apparatus noted above, it is also possible to
have it so that the print image generating unit has a printing head, and
while repeating a main scan cycle of the printing head N times (N is an
integer of 2 or more), generates a print image by forming dots on each of
the print pixels according to the dot data, and the plurality of pixel
position groups includes a plurality of pixel position groups divided
according to the remainder from the numerical value representing the
order of the Sub-scan direction of the main scan line divided by the
aforementioned N.

[0027] By doing this, it is possible to constitute a halftone process with
a high level of robustness to displacement in the Sub-scan direction for
interlace printing which embeds the main scan with a plurality of cycles.

[0028] With the printing apparatus noted above, it is also possible to
have it so that the print image generating unit has a plurality of
printing heads, and while performing the main scan of the plurality of
printing heads, generates a print image by forming dots on each of the
print pixels according to the dot data, and the plurality of pixel
position groups includes a plurality of pixel position groups in charge
of the dot formation by each of the plurality of printing heads.

[0029] By doing this, for printing using a plurality of printing heads, it
is possible to constitute a halftone process with a high level of
robustness to displacement of the dot formation position between mutual
printing heads, for example.

[0030] With the printing apparatus noted above, it is also possible to
have it so that the print image generating unit has a plurality of
printing heads, and while performing the Sub-scan of the print medium,
generates a print image by forming dots on each of the print pixels
according to the dot data, and the plurality of pixel position groups
includes a plurality of pixel position groups in charge of the dot
formation by each of the plurality of printing heads.

[0031] By doing this, it is possible to constitute the halftone process to
have a high level of robustness suitable for a line printer that forms
dots on each of the print pixels while performing the Sub-scan of the
print medium.

[0032] With the printing apparatus noted above, it is also possible to
have it so that the dot data generating unit has a dither matrix for
which a threshold value is set for each pixel, and the presence or
absence of dot formation for each of the print pixels is decided
according to the tone value of each pixel that constitutes the original
image and the threshold value set for the corresponding pixel position of
the dither matrix, and the dither matrix is constituted so that each of
the spatial frequency distributions of the threshold value set for the
pixels belonging to each of the plurality of pixel position groups and
the spatial frequency distributions of the print image have a mutually
positive correlation coefficient, or the dot data generating unit has a
dither matrix for which a threshold value is set for each pixel, and the
presence or absence iof dot formation for each of the print pixels is
decided according to the tone value of each pixel that constitutes the
original image and the threshold value set for the corresponding pixel
position of the dither matrix, and the dither matrix is constituted so
that each of the spatial frequency distributions of the threshold value
set for the pixels belonging to each of the plurality of pixel position
groups and the spatial frequency distributions of the print image have a
mutual correlation coefficient of 0.7 or greater.

[0033] If this kind of dither matrix is used, a significant effect is not
given to the spatial frequency distribution of the dots formed even when
the physical differences described above occur, so it is possible to
constitute a halftone process with a high level of robustness to the
physical differences described above.

[0034] With the printing apparatus noted above, it is also possible to
have it so that the halftone process is constituted so that each of the
spatial frequency distributions of the dot pattern formed on the print
pixels belonging to each of the plurality of pixel position groups and
the spatial frequency distributions of the print image have a mutually
positive correlation coefficient or the halftone process is constituted
so that each of the spatial frequency distributions of the dot pattern
formed on the print pixels belonging to each of the plurality of pixel
position groups and the spatial frequency distributions of the print
image have a mutual correlation coefficient of 0.7 or greater.

[0035] With this specification, "correlation coefficient" means the
Pearson's product moment correlation coefficient generally used as a
correlation coefficient. The Pearson's product moment correlation
coefficient is one statistical index indicating the correlation (degree
of similarity) between two data strings, and taking a real number value
from -1 to 1, when close to 1, this means that there is a correlation
between the two data strings, and when close to -1, it means there is a
negative correlation. When close to 0, it means the correlation of the
two data strings is weak. A correlation coefficient of 0.7 or greater
generally means that the correlation is strong to a degree that cannot
occur as a coincidental match.

[0036] The Pearson's product moment correlation coefficient is found by
dividing the covariance of the two data strings by the product of the two
data string standard deviation. That is to say, this can also be called
the value normalized to from -1 to 1 by dividing the covariance of the
two data strings by the product of the two data string standard
deviation. With the invention of this application, the two data strings
correlate to any two items selected from among a plurality of data
strings for which the spatial frequency distribution of each dot pattern
is digitized.

[0037] Note that the fact that when this invention is reliably mounted to
a printing apparatus an effect is exhibited can be confirmed using a
verification method for statistical engineering, for example. This
verification method calculates the probability of a null hypothesis (this
invention not mounted) occurring, and when that probability is low to a
certain degree, it is judged that the null hypothesis is mistaken, and
instead, a means of supporting an alternative hypothesis (this invention
is mounted) is used to proceed. Here, the probability that is the
criterion for judging that a null hypothesis is mistaken (significant
level) is decided as a design quality assurance demand item. By doing
this, it is possible to realize design quality assurance without
performing confirming for all the tones or colors.

[0038] In specific terms, it is possible to perform design quality
assurance of reliably mounting this invention on a printing apparatus
using the kind of method described below and exhibiting an effect, for
example.

[0039] (1) A sample of a specified number of gray tones for each pixel
position group is printed using a printing apparatus that is subject to
evaluation.

[0040] (2) The spatial frequency distribution is measured for each of the
printed patterns.

[0041] (3) The mutual correlation coefficient between the measured
plurality of spatial frequency distributions is found.

[0042] (4) It is confirmed that the correlation coefficient is positive or
0.7 or greater.

[0043] Here, the probability of a null hypothesis (this invention not
mounted) occurring decreases as the number of gray tone samples
increases.

[0044] Note that the correlation coefficient as a specified characteristic
as described previously does not absolutely have to be provided across
all the gradations reproduced by this halftone process, but can be
provided for part of the gradations. Here, "part of the gradations" is
preferably gradations for which the dot density is relatively low. This
is because with gradations for which the dot density is relatively low,
the dots stand out easily. In this kind of case, it is acceptable to
confirm the correlation coefficient for consecutive gradations for which
the dot density is relatively low, for example.

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] FIG. 1 is an explanatory drawing showing the summary of a printing
system as the printing apparatus of this embodiment;

[0046] FIG. 2 is an explanatory drawing showing the constitution of a
computer as the image processing device of this embodiment;

[0047] FIG. 3 is an explanatory drawing showing the schematic structure of
the color printer of this embodiment;

[0048] FIG. 4 is an explanatory drawing showing an array of inkjet nozzles
for an ink spray head;

[0049] FIG. 5 is a flow chart showing the flow of the image printing
process of this embodiment;

[0051] FIG. 7 is an explanatory drawing conceptually showing an example of
part of a dither matrix;

[0052] FIG. 8 is an explanatory drawing conceptually showing the state of
deciding the presence or absence of dot formation for each pixel while
referencing the dither matrix;

[0053] FIG. 9 is an explanatory drawing showing the findings that became
the beginning of the invention of this application;

[0054] FIG. 10 is an explanatory drawing conceptually showing an example
the spatial frequency characteristics of threshold values set for each
pixel of the dither matrix having blue noise characteristics;

[0055] FIGS. 11(A), 11(B), and 11(C) are explanatory drawings conceptually
showing the sensitivity characteristics VTF for the spatial frequency of
the visual sense that humans have;

[0056] FIGS. 12(A), 12(B), and 12(C) are explanatory drawings showing the
results of studying the granularity index of forward scan images for
various dither matrixes having blue noise characteristics;

[0057] FIGS. 13(A) and 13(B) are explanatory drawings showing the results
of studying the correlation coefficient between the position misalignment
image granularity index and the forward scan image granularity index;

[0058] FIG. 14 is an explanatory drawing showing the principle of it being
possible to suppress the image quality degradation even when dot position
misalignment occurs during bidirectional printing;

[0059] FIG. 15 is an explanatory drawing showing the degradation of image
quality due to presence or absence of dot position misalignment with
images formed using a general dither matrix;

[0060] FIG. 16 is a flow chart showing the flow of the process of
generating a dither matrix referenced with the tone number conversion
process of this embodiment;

[0061] FIGS. 17(A) and 17(B) are explanatory drawings showing the reason
that it is possible to ensure image quality during the occurrence of dot
position misalignment by not allowing mixing of first pixel positions and
second pixel positions within the same raster;

[0062] FIG. 18 is an explanatory drawing showing the printing status by
line printer 200L having printing heads 251 and 252 for the first
variation example of the invention;

[0063] FIGS. 19(A) and 19(B) are explanatory drawings showing the printing
status using the interlace recording method for the second variation
example of the invention;

[0064] FIG. 20 is an explanatory drawing showing the printing status using
the overlap recording method for the third variation example of the
invention;

[0065] FIG. 21 is an explanatory drawing showing a group of eight pixel
positions classified according to the number of remainders when the path
number is divided by 8;

[0066] FIGS. 22(A), 22(B), and 22(C) are is an explanatory drawing showing
an example of the actual printing status for the bidirectional printing
method of the fourth variation example of the invention; and

[0067] FIG. 23 is an explanatory drawing showing the state of the printing
image being formed with mutually combining four pixel position groups in
a common printing area in a case when conventional halftone processing
was performed.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0068] The present invention is explained in the following sequence based
on embodiments.

A. Summary of the Embodiment:

B. Device Constitution:

C. Summary of the Image Printing Process:

[0069] D. Principle of Suppressing Degradation of Image Quality Due to Dot
Position misalignment:

E. Dither Matrix Generating Method:

F. Variation Examples:

A. Summary of the Embodiments

[0070] Before starting the detailed description of the embodiment, a
summary of the embodiment is described while referring to FIG. 1. FIG. 1
is an explanatory drawing showing a summary of a printing system as the
printing apparatus of this embodiment. As shown in the drawing, the
printing system consists of a computer 10 as the image processing device,
a printer 20 that prints the actual images under the control of the
computer 10 and the like, and entire system is unified as one and
functions as a printing apparatus.

[0071] A dot formation presence or absence decision module and a dither
matrix are provided in the computer 10, and when the dot formation
presence or absence decision module receives image data of the image to
be printed, while referencing the dither matrix, data (dot data) is
generated that represents the presence or absence of dot formation for
each pixel, and the obtained dot data is output toward the printer 20.

[0072] A dot formation head 21 that forms dots while moving back and forth
over the print medium and a dot formation module that controls the dot
formation at the dot formation head 21 are provided in the printer 20.
When the dot formation module receives dot data output from the computer
10, dot data is supplied to the head to match the movement of the dot
formation head 21 moving back and forth. As a result, the dot formation
head 21 that moves back and forth over the print medium is driven at a
suitable timing, forms dots at suitable positions on the print medium,
and an image is printed.

[0073] Also, with the printing apparatus of this embodiment, by performing
so called bidirectional printing for which dots are formed not only
during forward scan of the dot formation head 21 but also during backward
scan, it is possible to rapidly print images. It makes sense that when
performing bidirectional printing, when dot formation position
misalignment occurs between dots formed during forward scan and dots
formed during backward scan, the image quality is degraded. In light of
this, it is normal to have built into this kind of printer a special
mechanism or control for adjusting at a high precision the timing of dot
formation of one of the back and forth movements to the other timing, and
this is one factor in causing printers to be larger or more complex.

[0074] Considering this kind of point, with the printing apparatus of this
embodiment shown in FIG. 1, as the dither matrix referenced when
generating dot data from the image data, a matrix having at least the
following two characteristics is used. Specifically, as the first
characteristic, this is a matrix for which it is possible to classify the
dither matrix pixel positions into a first pixel position group and a
second pixel position group. Here, the first pixel position and the
second pixel position are pixel positions having a relationship whereby
when one has dots formed at either the forward scan or the backward scan,
the other has dots formed at the opposite. Then as the second
characteristic, this is a matrix for which the dither matrix, a matrix
for which the threshold values set for the first pixel positions are
removed from the dither matrix (first pixel position matrix), and a
matrix for which the threshold values set for the second pixel positions
are removed (second pixel position matrix) all have blue noise
characteristics.

[0075] Here, though the details are described later, the inventors of this
application discovered the following kind of new findings. Specifically,
there is a very strong correlation between the image quality of images
for which the dot formation position was displaced between the forward
scan and the backward scan and the image quality of images made only by
dots formed during forward scan (images obtained with only the dots
formed during the backward scan removed from the original image;
hereafter called "forward scan images"), or the image quality of images
made only by dots formed during backward scan (images obtained with only
the dots formed during the forward scan removed from the original image;
hereafter called "backward scan images"). Then, if the image quality of
the forward scan images or the image quality of the backward scan images
is improved, even when dot formation position misalignment occurs between
the forward scan and the backward scan of bidirectional printing, it is
possible to suppress degradation of image quality. Therefore, the dither
matrix can be classified by the characteristics noted above,
specifically, it is possible to classify as a first pixel position matrix
and a second pixel position matrix, and if dot data is generated using a
dither matrix such as one for which these three matrixes have blue noise
characteristics, it is possible to have both the forward scan images and
the backward images be good image quality images, so it is possible to
suppress to a minimum the degradation of image quality even when there is
dot formation position misalignment during bidirectional printing. As a
result, when adjusting the dot formation timing of one of the back and
forth movements to the other timing, there is no demand for high
precision, so it is possible to have a simple mechanism and control for
adjustment, and thus, it is possible to avoid the printer becoming large
and complex. Following, this kind of embodiment is described in detail.

B. Device Constitution

[0076] FIG. 2 is an explanatory drawing showing the constitution of the
computer 100 as the image processing device of this embodiment. The
computer 100 is a known computer constituted by a CPU 102 as the core, a
ROM 104, a RAM 106 and the like being mutually connected by a bus 116.

[0077] Connected to the computer 100 are a disk controller DDC 109 for
reading data of a flexible disk 124, a compact disk 126 or the like, a
peripheral device interface PIF 108 for performing transmission of data
with peripheral devices, a video interface VIF 112 for driving a CRT 113,
and the like. Connected to the PIF 108 are a color printer 200 described
later, a hard disk 118, or the like. Also, if a digital camera 120 or
color scanner 122 or the like is connected to the PIF 108, it is possible
to perform image processing on images taken by the digital camera 120 or
the color scanner 122. Also, if a network interface card NIC 110 is
mounted, the computer 100 is connected to the communication line 300, and
it is possible to fetch data stored in the storage device 310 connected
to the communication line. When the computer 100 fetches image data of
the image to be printed, by performing the specified image processing
described later, the image data is converted to data representing the
presence or absence of dot formation for each pixel (dot data), and
output to the color printer 200.

[0078] FIG. 3 is an explanatory drawing showing the schematic structure of
the color printer 200 of this embodiment. The color printer 200 is an ink
jet printer capable of forming dots of four colors of ink including cyan,
magenta, yellow, and black. Of course, in addition to these four colors
of ink, it is also possible to use an inkjet printer capable of forming
ink dots of a total of six colors including an ink with a low dye or
pigment concentration of cyan (light cyan) and an ink with a low dye or
pigment concentration of magenta (light magenta). Note that following, in
some cases, cyan ink, magenta ink, yellow ink, black ink, light cyan ink,
and light magenta ink are respectively called C ink, M ink, Y ink, K ink,
LC ink, and LM ink.

[0079] As shown in the drawing, the color printer 200 consists of a
mechanism that drives a printing head 241 built into a carriage 240 and
performs blowing of ink and dot formation, a mechanism that moves this
carriage 240 back and forth in the axial direction of a platen 236 by a
carriage motor 230, a mechanism that transports printing paper P by a
paper feed motor 235, a control circuit 260 that controls the dot
formation, the movement of the carriage 240 and the transport of the
printing paper, and the like.

[0080] Mounted on the carriage 240 are an ink cartridge 242 that holds K
ink, and an ink cartridge 243 that holds each type of ink C ink, M ink,
and Y ink. When the ink cartridges 242 and 243 are mounted on the
carriage 240, each ink within the cartridge passes through an
introduction tube that is not illustrated and is supplied to each color
ink spray heads 244 to 247 provided on the bottom surface of the printing
head 241.

[0081] FIG. 4 is an explanatory drawing showing an array of inkjet nozzle
Nz for the ink spray heads 244 to 247. As shown in the drawing, on the
bottom surface of the ink spray heads are formed four sets of nozzle
arrays that spray each color of ink C, M, Y, and K, and 48 nozzles Nz per
one set of nozzle arrays are arranged at a fixed nozzle pitch k.

[0082] The control circuit 260 of the color printer 200 is constituted by
a CPU, ROM, RAM, PIF (peripheral device interface), and the like mutually
connected by a bus, and by controlling the operation of the carriage
motor 230 and the paper feed motor 235, it controls the main scan
movement and Sub-scan movement of the carriage 240. Also, when the dot
data output from the computer 100 is received, by supplying dot data to
the ink spray heads 244 to 247 to match the main scan or Sub-scan
movement of the carriage 240, it is possible to drive these heads.

[0083] The color printer 200 having the kind of hardware constitution
noted above, by driving the carriage motor 230, moves each color ink
spray head 244 to 247 back and forth in the main scan direction, and by
driving the paper feed motor 235, moves the printing paper P in the
Sub-scan direction. The control circuit 260, by driving the nozzles at a
suitable timing based on dot data to match the back and forth movement of
the carriage 240 (main scan) and the paper feed movement of the print
medium (Sub-scan), forms suitable colored ink dots at suitable positions
on the print medium. By working in this way, the color printer 200 is
able to print color images on the printing paper.

[0084] Note that though the printer of this embodiment was described as a
so called inkjet printer that forms ink dots by spraying ink drops toward
a print medium, it can also be a printer that forms dots using any
method. For example, the invention of this application, instead of
spraying ink drops, can also be suitably applied to a printer that forms
dots by adhering each color of toner powder onto the print medium using
static electricity, or a so called dot impact method printer.

C. Summary of the Image Printing Process

[0085] FIG. 5 is a flow chart showing the process flow of adding a
specified image process by the computer 100 to an image to be printed,
converting image data to dot data expressed by the presence or absence of
dot formation, supplying to the color printer 200 as control data the
obtained dot data, and printing the image.

[0086] When the computer 100 starts image processing, first, it starts
reading the image data to be converted (step S100). Here, the image data
is described as RGB color image data, but it is not limited to color
image data, and it is also possible to apply this in the same way for
black and white image data as well.

[0087] After reading of the image data, the resolution conversion process
is started (step S102). The resolution conversion process is a process
that converts the resolution of the read image data to resolution
(printing resolution) at which the color printer 200 is to print the
image. When the print resolution is higher than the image data
resolution, an interpolation operation is performed and new image data is
generated to increase the resolution. Conversely, when the image data
resolution is higher than the printing resolution, the resolution is
decreased by culling the read image data at a fixed rate. With the
resolution conversion process, by performing this kind of operation on
the read image data, the image data resolution is converted to the
printing resolution.

[0088] Once the image data resolution is converted to the printing
resolution in this way, next, color conversion processing is performed
(step S104). Color conversion processing is a process of converting RGB
color image data expressed by a combination of R, G, and B tone values to
image data expressed by combinations of tone values of each color used
for printing. As described previously, the color printer 200 prints
images using four colors of ink C, M, Y, and K. In light of this, with
the color conversion process of this embodiment, the image data expressed
by each color RGB undergoes the process of conversion to data expressed
by the tone values of each color C, M, Y, and K.

[0089] The color conversion process is able to be performed rapidly by
referencing a color conversion table (LUT). FIG. 6 is an explanatory
drawing that conceptually shows the LUT referenced for color conversion
processing. The LUT can be thought of as a three dimensional number chart
if thought of in the following way. First, as shown in FIG. 6, we think
of a color space using three orthogonal axes of the R axis, the G axis,
and the B axis. When this is done, all the RGB image data can definitely
be displayed correlated to coordinate points within the color space. From
this, if the R axis, the G axis, and the B axis are respectively
subdivided and a large number of grid points are set within the color
space, each of the grid points can be thought of as representing the RGB
image data, and it is possible to correlate the tone values of each color
C, M, Y, and K corresponding to each RGB image data to each grid point.
The LUT can be thought of as a three dimensional number chart in which is
correlated and stored the tone values of each color C, M, Y, and K to the
grid points provided within the color pace in this way. If color
conversion processing is performed based on the correlation of RGB color
image data and tone data of each color C, M, YU, and K stored in this
kind of LUT, it is possible to rapidly convert RGB color image data to
tone data of each color C, M, Y, and K.

[0090] When tone data of each color C, M, Y, and K is obtained in this
way, the computer 100 starts the tone number conversion process (step
S106). The tone number conversion process is the following kind of
process. The image data obtained by the color conversion process, if the
data length is 1 byte, is tone data for which values can be taken from
tone value 0 to tone value 255 for each pixel. In comparison to this, the
printer displays images by forming dots, so for each pixel, it is only
possible to have either state of "dots are formed" or "dots are not
formed." In light of this, instead of changing the tone value for each
pixel, with this kind of printer, images are expressed by changing the
density of dots formed within a specified area. The tone number
conversion process is a process that, to generate dots at a suitable
density according to the tone value of the tone data, decides the
presence or absence of dot formation for each pixel.

[0091] As a method of generating dots at a suitable density according to
the tone values, various methods are known such as the error diffusion
method and the dither method, but with the Tone number conversion process
of this embodiment, the method called the dither method is used. The
dither method of this embodiment is a method that decides the presence or
absence of dot formation for each pixel by comparing the threshold value
set in the dither matrix and the tone value of the image data for each
pixel. Following is a simple description of the principle of deciding on
the presence or absence of dot formation using the dither method.

[0092] FIG. 7 is an explanatory drawing that conceptually shows an example
of part of a dither matrix. The matrix shown in the drawing randomly
stores threshold values selected thoroughly from a tone value range of 1
to 255 for a total of 8192 pixels, with 128 pixels in the horizontal
direction (main scan direction) and 64 pixels in the vertical direction
(Sub-scan direction). Here, selecting from a range of 1 to 255 for the
tone value of the threshold value with this embodiment is because in
addition to having the image data as 1 byte data that can take tone
values from values 0 to 255, when the image data tone value and the
threshold value are equal, it is decided that a dot is formed at that
pixel.

[0093] Specifically, when dot formation is limited to pixels for which the
image data tone value is greater than the threshold value (specifically,
dots are not formed on pixels for which the tone value and threshold
value are equal), dots are definitely not formed at pixels having
threshold values of the same value as the largest tone value that the
image data can have. To avoid this situation, the range that the
threshold values can have is made to be a range that excludes the maximum
tone value from the range that the image data can have. Conversely, when
dots are also formed on pixels for which the image data tone value and
the threshold value are equal, dots are always formed at pixels having a
threshold value of the same value as the minimum tone value that the
image data has. To avoid this situation, the range that the threshold
values can have is made to be a range excluding the minimum tone value
from the range that the image data can have. With this embodiment, the
tone values that the image data can have is from 0 to 255, and since dots
are formed at pixels for which the image data and the threshold value are
equal, the range that the threshold values can have is set to 1 to 255.
Note that the size of the dither matrix is not limited to the kind of
size shown by example in FIG. 7, but can also be various sizes including
a matrix for which the vertical and horizontal pixel count is the same.

[0094] FIG. 8 is an explanatory drawing that conceptually shows the state
of deciding the presence or absence of dot formation for each pixel while
referring to the dither matrix. When deciding on the presence or absence
of dot formation, first, a pixel for deciding about is selected, and the
tone value of the image data for that pixel and the threshold value
stored at the position corresponding in the dither matrix are compared.
The fine dotted line arrow shown in FIG. 8 typically represents the
comparison for each pixel of the tone value of the image data and the
threshold value stored in the dither matrix. For example, for the pixel
in the upper left corner of the image data, the threshold value of the
image data is 97, and the threshold value of the dither matrix is 1, so
it is decided that dots are formed at this pixel. The arrow shown by the
solid line in FIG. 8 typically represents the state of it being decided
that dots are formed in this pixel, and of the decision results being
written to memory. Meanwhile, for the pixel that is adjacent at the right
of this pixel, the tone value of the image data is 97, and the threshold
value of the dither matrix is 177, and since the threshold value is
larger, it is decided that dots are not formed at this pixel, With the
dither method, by deciding whether or not to form dots for each pixel
while referencing the dither matrix in this way, image data is converted
to data representing the presence or absence of dot formation for each
pixel. In this way, if using the dither method, it is possible to decide
the presence or absence of dot formation for each pixel with a simple
process of comparing the tone value of the image data and the threshold
value set in the dither matrix, so it is possible to rapidly implement
the tone number conversion process.

[0095] Also, when the image data tone value is determined, as is clear
from the fact that whether or not dots are formed on each pixel is
determined by the threshold value set in the dither matrix, with the
dither method, it is possible to actively control the dot generating
status by the threshold value set in the dither matrix. With the tone
number conversion process of this embodiment, using this kind of feature
of the dither method, by deciding on the presence or absence of dot
formation for each pixel using the dither matrix having the special
characteristics described later, even in cases when there is dot
formation position misalignment between dots formed during forward scan
and dots formed during backward scan when doing bidirectional printing,
it is possible to suppress to a minimum the degradation of image quality
due to this. The principle of being able to suppress to a minimum the
image quality degradation and the characteristics provided with a dither
matrix capable of this are described in detail later.

[0096] When the tone number conversion process ends and data representing
the presence or absence of dot formation for each pixel is obtained from
the tone data of each color C, M, Y, and K, this time, the interlace
process starts (step S108). The interlace process is a process that
realigns the sequence of transfer of image data converted to the
expression format according to the presence or absence of dot formation
to the color printer 200 while considering the sequence by which dots are
actually formed on the printing paper. The computer 100, after realigning
the image data by performing the interlace process, outputs the finally
obtained data as control data to the color printer 200 (step S110).

[0097] The color printer 200 prints images by forming dots on the printing
paper according to the control data supplied from the computer 100 in
this way. Specifically, as described previously using FIG. 3, the main
scan and the Sub-scan of the carriage 240 are performed by driving the
carriage motor 230 and the paper feed motor 235, and the head 241 is
driven based on the dot data to match these movements, and ink drops are
sprayed. As a result, suitable color ink dots are formed at suitable
positions and an image is printed.

[0098] The color printer 200 described above forms dots while moving the
carriage 240 back and forth to print images, so if dots are formed not
only during the forward scan of the carriage 240 but also during the
backward scan, it is possible to rapidly print images. It makes sense
that when performing this kind of bidirectional printing, when dot
formation position misalignment occurs between dots formed during the
forward scan of the carriage 240 and the dots formed during the backward
scan, the image quality will be degraded. In light of this, to avoid this
kind of situation, a normal color printer is made to be able to adjust
with good precision the timing of forming dots for at least one of during
forward scan or backward scan. Because of this, it is possible to match
the position at which dots are formed during the forward scan and the
position at which dots are formed during the backward scan, and it is
possible to rapidly print images with high image quality without
degradation of the image quality even when bidirectional printing is
performed. However, on the other hand, because it is possible to adjust
with good precision the timing of forming dots, a dedicated adjustment
mechanism or adjustment program is necessary, and there is a tendency for
the color printer to become more complex and larger.

[0099] To avoid the occurrence of this kind of problem, with the computer
100 of this embodiment, even when there is a slight displacement of the
dot formation position during the forward scan and the backward scan, the
presence or absence of dot formation is decided using a dither matrix
that makes it possible to suppress to a minimum the effect on image
quality. If the presence or absence of dot formation for each pixel is
decided by referencing this kind of dither matrix, even if there is
slight displacement of the dot formation positions between the forward
scan and the backward scan, there is no significant effect on the image
quality. Because of this, it is not necessary to adjust with high
precision the dot formation position, and it is possible to use simple
items for the mechanism and control contents for adjustment, so it is
possible to avoid the color printer from becoming needlessly large and
complex. Following, the principle that makes this possible is described,
and after that, a simple description is given of one method for
generating this kind of dither matrix.

D. Principle of Suppressing Degradation of Image Quality Due to Dot
Position Misalignment

[0100] The invention of this application was completed with the discovery
of new findings regarding images formed using the dither matrix as the
beginning. In light of this, first, the findings we newly discovered as
the beginning of the invention of this application are explained.

[0101] FIG. 9 is an explanatory drawing showing the findings that became
the beginning of the invention of this application. Overall dot
distribution Dpall shows an expanded view of the state of dots being
formed at a specified density for forming images of certain tone values.
As shown in Overall dot distribution Dpall, to obtain the optimal image
quality image, it is necessary to form dots in a state dispersed as
thoroughly as possible.

[0102] To form dots in a thoroughly dispersed state in this way, it is
known that it is possible to reference a dither matrix having so-called
blue noise characteristics to decide the presence or absence of dot
formation. Here, a dither matrix having blue noise characteristics means
a matrix like the following. Specifically, it means a dither matrix for
which while dots are formed irregularly, the spatial frequency component
of the set threshold value has the largest component in a high frequency
range for which one cycle is two pixels or less. Note that bright (high
brightness level) images and the like can also be cases when dots are
formed in regular patterns near a specific brightness level.

[0103] FIG. 10 is an explanatory drawing that conceptually shows an
example of the spatial frequency characteristics of the threshold values
set for each pixel of a dither matrix having blue noise characteristics
(following, this may also be called a blue noise matrix). Note that with
FIG. 10, in addition to the blue noise matrix spatial frequency
characteristics, there is also a display regarding the spatial frequency
characteristics of the threshold values set in a dither matrix having so
called green noise characteristics (hereafter, this is also called a
green noise matrix). The green noise matrix spatial frequency
characteristics will be described later, but first, the blue noise matrix
spatial frequency characteristics are described.

[0104] In FIG. 10, due to circumstances of display, instead of using
spatial frequency for the horizontal axis, cycles are used. It goes
without saying that the shorter the cycle, the higher the spatial
frequency. Also, the vertical axis of FIG. 10 shows the spatial frequency
component for each of the cycles. Note that the frequency components
shown in the drawing indicate a state of being smoothed so that the
changes are smooth to a certain degree.

[0105] The spatial frequency component of the threshold values set for the
blue noise matrix is shown by example using the solid line in the
drawing. As shown in the drawing, the blue noise matrix spatial frequency
characteristics are characteristics having the maximum frequency
component in the high frequency range for which one cycle length is two
pixels or less. The threshold values of the blue noise matrix are set to
have this kind of spatial frequency characteristics, so if the presence
or absence of dot formation is decided based on a matrix having this kind
of characteristics, then dots are formed in a state separated from each
other.

[0106] From the kinds of reasons described above, if the presence or
absence of dot formation for each pixel is decided while referencing a
dither matrix having blue noise characteristics, as shown in the Overall
dot distribution Dpall, it is possible to obtain an image with thoroughly
dispersed dots. Conversely, because dots are generated dispersed
thoroughly as shown in the Overall dot distribution Dpall, threshold
values adjusted so as to have blue noise characteristics are set in the
dither matrix.

[0107] Note that here, the spatial frequency characteristics of the
threshold values set in the green noise matrix shown in FIG. 10 are
described. The dotted line curve shown in FIG. 10 shows an example of
green noise matrix spatial frequency characteristics. As shown in the
drawing, green noise matrix spatial frequency characteristics are
characteristics having the largest frequency component in the medium
frequency range for which the length of one cycle is from two pixels to
ten or more pixels. The green noise matrix threshold values are set so as
to have this kind of spatial frequency characteristics, so when the
presence or absence of dot formation for each pixel is decided while
referencing a dither matrix having green noise characteristics, while
dots are formed adjacent in several dot units, overall, the dot group is
formed in a dispersed state. As with a so-called laser printer or the
like, with a printer for which stable formation of fine dots of
approximately one pixel is difficult, by deciding the presence or absence
of dot formation while referencing this kind of green noise matrix, it is
possible to suppress the occurrence of isolated dots. As a result, it
becomes possible to rapidly output images with stable image quality.
Conversely, threshold values adjusted to have green noise characteristics
are set in the dither matrix referenced when deciding the presence or
absence of dot formation with a laser printer or the like.

[0108] As described above, with an inkjet printer like the color printer
200, a dither matrix having blue noise characteristics is used, and
therefore, as shown in the Overall dot distribution Dpall, the obtained
image is an image with thoroughly dispersed dots. However, when this
image is viewed with the dots formed during forward scan of the head
separated from the dots formed during the backward scan, we found that
the images made only by dots formed during the forward scan (forward scan
images) and the images made only by dots formed during the backward scan
(backward scan images) do not necessarily have the dots thoroughly
dispersed. Dots formed during forward scan Dpf is an image obtained by
extracting only the dots formed during the forward scan from the image
shown in the Overall dot distribution Dpall. Also, Dots formed during
backward scan Dpb is an image obtained by extracting only the dots formed
during the backward scan from the image shown in the Overall dot
distribution Dpall.

[0109] As shown in the drawing, if the dots formed by both the back and
forth movements are matched, as shown in the Overall dot distribution
Dpall, regardless of the fact that the dots are formed thoroughly, the
image of only the dots formed during the forward scan shown in the dots
formed during forward scan Dpf or the image of only the dots formed
during the backward scan shown in the dots formed during backward scan
Dpb are both generated in a state with the dots unbalanced.

[0110] In this way, though it is unexpected that there would be a big
difference in tendency, if we think in the following way, it seems that
this is a phenomenon that occurs half by necessity. Specifically, as
described previously, the dot distribution status depends on the setting
of the threshold values of the dither matrix, and the dither matrix
threshold values are set with special generation of the distribution of
the threshold values to have blue noise characteristics so that the dots
are dispersed well. Here, among the dither matrix threshold values,
threshold values of pixels for which dots are formed during the forward
scan or threshold values of pixels for which dots are formed during the
backward scan are taken, and with no consideration such has having the
distribution of the respective threshold values having blue noise
characteristics, the fact that the distribution of these threshold
values, in contrast to the blue noise characteristics, have
characteristics having a large frequency component in the long frequency
range, seems half necessary (see FIG. 10). Also, for a dither matrix
having green noise characteristics as well, when we consider that this is
a matrix specially set for the threshold value distribution to have green
noise characteristics, the threshold values of the pixels for which dots
are formed during the forward scan or the backward scan are considered to
have a large frequency component on a longer cycle side than the cycle
for which the green noise matrix has a large frequency component (see
FIG. 10). In the end, when the threshold values of pixels for which dots
are formed during the forward scan or the threshold values of pixels for
which dots are formed during the backward scan are taken from the dither
matrix having blue noise characteristics, the distribution of those
threshold values have large frequency components in the Visually
sensitive range. Because of this, for example, even when images have dots
thoroughly dispersed, when only dots formed during the forward scan or
only dots formed during the backward scan are removed, the obtained
images respectively are considered to be images for which the dots have
unbalance occur such as shown in the dots formed during forward scan Dpf
and the dots formed during backward scan Dpb. Specifically, the
phenomenon shown in FIG. 9 is not a special phenomenon that occurs with a
specific dither matrix, but rather can be thought of as the same
phenomenon that occurs with most dither matrixes.

[0111] Considering the kind of new findings noted above and the
considerations for these findings, studies were done for other dither
matrixes as well. With the study, to quantitatively evaluate the results,
an index called the granularity index was used. In light of this, before
describing the study results, we will give a brief description of the
granularity index.

[0112] FIG. 11 is an explanatory drawing that conceptually shows the
sensitivity characteristics VTF (Visual Transfer Function) to the visual
spatial frequency that humans have. As shown in the drawing, human vision
has a spatial frequency showing a high sensitivity, and there is a
characteristic of the sensitivity decreasing gradually as the spatial
frequency increases. It is also known that there is a characteristic of
the vision sensitivity decreasing also in ranges for which the spatial
frequency is extremely low. An example of this kind of human vision
sensitivity characteristic is shown in FIG. 11 (a). Various experimental
formulae have been proposed as an experimental formula for giving this
kind of sensitivity characteristic, but a representative experimental
formula is shown in FIG. 11 (b). Note that the variable L in FIG. 11 (b)
represents the observation distance, and the variable u represents the
spatial frequency.

[0113] Based on this kind of visual sensitivity characteristic VTF, it is
possible to think of a granularity index (specifically, an index
representing how easy it is for a dot to stand out). Now, we will assume
that a certain image has been Fourier transformed to obtain a power
spectrum. If that power spectrum happens to contain a large frequency
component, that doesn't necessarily mean that that image will immediately
be an image for which the dots stand out. This is because as described
previously using FIG. 11 (a), if that frequency is in the low range of
human visual sensitivity, for example even if it has a large frequency
component, the dots do not stand out that much. Conversely, with
frequencies in the high range of human visual sensitivity, for example
even when there are only relatively low frequency components, for the
entity doing the viewing, there are cases when the dots are sensed to
stand out. From this fact, the image is Fourier transformed to obtain a
power spectrum FS, the obtained power spectrum FS is weighted to
correlate to the human visual sensitivity characteristic VTF, and if
integration is done with each spatial frequency, then an index indicating
whether or not a human senses the dots as standing out or not is
obtained. The granularity index is an index obtained in this way, and can
be calculated by the calculation formula shown in FIG. 11 (c). Note that
the coefficient K in FIG. 11 (c) is a coefficient for matching the
obtained value with the human visual sense.

[0114] To confirm that the phenomenon described previously using FIG. 9 is
not a special phenomenon that occurs with a specific dither matrix, but
rather occurs also with most dither matrixes, the following kind of study
was performed on various dither matrixes having blue noise
characteristics. First, from among the dots formed by bidirectional
printing, images made only by dots formed during the forward scan such as
shown in the dots formed during forward scan Dpf (forward scan images)
are obtained. Next, the granularity index of the obtained images is
calculated. This kind of operation was performed for various dither
matrixes while changing the image tone values.

[0115] FIG. 12 is an explanatory drawing showing the results of studying
the granularity index of forward scan images for various dither matrixes
having blue noise characteristics. Shown in FIG. 12 are only the results
obtained for three dither matrixes with different resolutions. The dither
matrix A shown in FIG. 12 (a) is a dither matrix for printing at a main
scan direction resolution of 1440 dpi and Sub-scan direction resolution
of 720 dpi, and the dither matrix B shown in FIG. 21 (b) is a dither
matrix used for printing at a resolution of 1440 dpi for both the main
scan direction and the Sub-scan direction. Also, the dither matrix C
shown in FIG. 12 (c) is a dither matrix for printing in the main scan
direction at a resolution of 720 dpi and in the Sub-scan direction at a
resolution of 1440 dpi. Note that in FIG. 12, the horizontal axis is
displayed using the small dot formation density, and the areas for which
the displayed small dot formation density is 40% or less correlate to
areas up to before the intermediate gradation area from the highlight
area for which it is considered that the dots stand out relatively
easily.

[0116] Regardless of the fact that the three forward scan images shown in
FIG. 12 are generated from individually created dither matrixes for
printing respectively at different resolutions, each has an area for
which the granularity index is degraded (specifically, an area in which
the dots stand out easily). In this kind of area, the forward scan image
can be thought of as the dots generating imbalance as shown in the dots
formed during forward scan Dpf. In the end, all of the three dither
matrixes shown in FIG. 12 have blue noise characteristics, and therefore,
regardless of the fact that the images formed using bidirectional
printing have dots formed without imbalance, in at least part of the
gradation area, the forward scan image or the backward scan image has dot
imbalance occur. From this, the phenomenon described previously using
FIG. 9 can be thought of not as a special phenomenon that occurs with a
specific dither matrix but rather as a general phenomenon that occurs
with most dither matrixes. Then, when we consider the occurrence of dot
imbalance with either forward scan images or backward scan images in this
way, this can be thought of as possibly having an effect on the image
quality degradation due to dot position misalignment during bidirectional
printing. In light of this, we tried studying to see whether or not any
kind of correlation can be seen between the granularity index of images
formed with an intentional displacement in the dot formation position
during bidirectional printing (position misalignment image) and the
granularity index of forward scan images.

[0117] FIG. 13 is an explanatory drawing showing the results of studying
the correlation coefficient between the position misalignment image
granularity index and the forward scan image granularity index. FIG. 13
(a) shows the results of a study on the dither matrix A shown in FIG. 12
(a), and in the drawing, the black circles represent the position
misalignment image granularity index and the white circles in the drawing
represent the granularity index for the forward scan image. Also, FIG. 13
(b) shows the results of a study on the dither matrix B shown in FIG. 12
(b), and the black squares represent the position misalignment image
granularity index while the white squares represent the forward image
granularity index. As is clear from FIG. 13, for any of the dither
matrixes, a surprisingly strong correlation is seen between the position
misalignment image granularity index and the forward image granularity
index. From this fact, for the phenomenon of the image quality being
degraded by the dot position misalignment during bidirectional printing,
the fact that the bidirectional image dot imbalance becomes marked due to
displacement of the relative position between the forward scan images and
the backward scan images can be considered to be one significant factor.
Conversely, if the dot imbalance between the forward scan images and the
backward scan images is reduced, for example even when dot position
misalignment occurs during bidirectional printing, it is thought that it
is possible to suppress image quality degradation.

[0118] FIG. 14 is an explanatory drawing showing that it is possible to
suppress the image quality degradation when dot position misalignment
occurs during bidirectional printing if the dot imbalance is reduced for
images during forward scan and images during backward scan. Dot pattern
Dat and dot pattern Dmat show a comparison of an image for which
bidirectional printing was performed in a state without dot position
misalignment and an image printed in a state with intentional
displacement by a specified volume of the dot formation position. Also,
shown respectively in FIG. 14, Forward scan image Fsit and Backward scan
image Bsit are images obtained by breaking down into an image made only
by dots formed during the forward scan of the head (forward scan image)
and an image made only by dots formed during the backward scan (backward
scan image).

[0119] As shown in the forward scan image Fsit and the backward scan image
Bsit, the forward scan images and the backward scan images are both
images for which the dots are dispersed thoroughly. Also, as shown in the
forward scan image Fsit, in the state with no dot position misalignment,
images obtained by synthesizing the forward scan images and backward scan
images (specifically, images obtained with bidirectional printing) are
also images for which the dots are dispersed thoroughly. In this way, not
just images obtained by performing bidirectional printing, but also when
broken down into forward scan images and backward images, images that
have the dots dispersed thoroughly with the respective images can be
obtained by deciding the presence or absence of dot formation while
referencing a dither matrix having the kind of characteristics described
later in the tone number conversion process of FIG. 5. Then, the backward
scan image Bsit correlates to an image for which this kind of forward
scan image and backward scan image are overlapped in a state displaced by
a specified amount.

[0120] If the image without position misalignment (left side image) shown
in the forward scan image Fsit and the image with position misalignment
(right side image) are compared, by the dot position being displaced, the
right side image has its dots stand out slightly more easily than the
left side image with no displacement, but we can understand that this is
not at a level that greatly degrades the image quality. This is thought
to show that even when broken down into forward scan images and backward
scan images, if dots are generated so that the dots are dispersed
thoroughly, for example even when dot position misalignment occurs during
bidirectional printing, it is possible to greatly suppress degradation of
image quality due to this.

[0121] As a reference, with the image formed using a typical dither
matrix, we checked to what degree image quality degraded when dot
position misalignment occurred by the same amount as the case shown in
FIG. 14. FIG. 15 is an explanatory drawing showing degradation of the
image quality due to the presence or absence of dot position misalignment
with the image formed by a typical dither matrix. The image without
position misalignment (left side image) shown in Dot pattern Dar is an
image for which the forward scan image and backward scan image shown in
FIG. 9 are overlapped without any position misalignment. Also, the image
with position misalignment shown in Dot pattern Dar is an image for which
the forward scan image and the backward scan image are overlapped in a
state with the position displaced by the same amount as the case shown in
FIG. 14. Note that in the forward scan image Fsir and the backward scan
image Bsir, the respective forward scan images and backward scan images
are shown.

[0122] As is clear from FIG. 15, when dots are generated with imbalance
with the forward scan image and the backward scan image, it is possible
to confirm that when the dot formation positions are displaced during
bidirectional printing, there is great degradation of the image quality
when the image quality is greatly degraded [sic]. Also, when FIG. 14 and
FIG. 15 are compared, by thoroughly dispersing the dots with the forward
scan image and the backward scan image, it is possible to understand that
the image quality degradation due to dot position misalignment can be
dramatically improved.

[0123] With the color printer 200 of this embodiment, based on this kind
of principle, it is possible to suppress to a minimum the image quality
degradation due to dot position misalignment during bidirectional
printing. Because of this, during bidirectional printing, even when the
formation positions of the dots formed during forward scan and the dots
formed during backward scan are not matched with high precision, there is
no degradation of image quality. As a result, there is no need for a
mechanism or control program for adjusting with good precision the dot
position misalignment, so it is possible to use a simple constitution for
the printer. Furthermore, it is possible to reduce the precision required
for the mechanism for moving the head back and forth as well, and this
point also makes it possible to simplify the printer constitution.

E. Dither Matrix Generating Method

[0124] Next, a simple description is given of an example of a method of
generating a dither matrix to be referenced by the tone number conversion
process of this embodiment. Specifically, with the tone number conversion
process of this embodiment, for dots formed during the forward scan, for
dots formed during the backward scan, and furthermore, for combinations
of these dots, dots are generated in a thoroughly dispersed state, so
gradation conversion processing is performed while referencing a dither
matrix having the following two kinds of characteristics.

[0125] "First Characteristic": The dither matrix pixel positions can be
classified into first pixel position groups and second pixel position
groups. Here, the first pixel position and the second pixel position mean
pixel positions having a mutual relationship such that when dots are
formed by either the forward scan or the backward scan, the other has
dots formed by the other.

[0126] "Second Characteristic": The dither matrix and a matrix for which
the threshold values set for the first pixel position are removed from
that dither matrix (first pixel position matrix), and a matrix for which
the threshold values set for the second pixel positions are removed
(second pixel position matrix) all have either blue noise characteristics
or green noise characteristics. Here, a "dither matrix having blue noise
characteristics" means the following kind of matrix. Specifically, it
means a dither matrix for which dots are generated irregularly and the
spatial frequency component of the set threshold values have the largest
component in the medium frequency range for which one cycle is from two
pixels to ten or more pixels. Also, a "dither matrix having green noise
characteristics" means a dither matrix for which dots are formed
irregularly and the spatial frequency component of the set threshold
values have the largest component in the medium frequency range for which
one cycle has from two pixels to ten or more pixels. Note that if these
dither matrixes are near a specific brightness, it is also acceptable if
there are dots formed in a regular pattern.

[0127] As described previously, dither matrixes having these kind of
characteristics can definitely not be generated by coincidence, so a
brief description is given of an example of a method for generating this
kind of dither matrix.

[0128] FIG. 16 is a flow chart showing the flow of the process of
generating dither matrixes referenced with the tone number conversion
process of this embodiment. Note that here, with an existing dither
matrix having blue noise characteristics as a source, so that the "first
characteristics" and "second characteristics" described above can be
obtained, described is a method to which correction is added. It makes
sense that rather than correcting the matrix that is the source, that it
is also possible to generate first from a dither matrix having the "first
characteristics" and "second characteristics." Also, here, described is a
case when a matrix having blue noise characteristics is the source, but
it is also possible to obtain a dither matrix having the characteristics
noted above by working in about the same manner when using a dither
matrix having green noise characteristics as the source as well.

[0129] When the dither matrix generating process starts, first, the dither
matrix that is the source is read (step S200). This matrix overall has
blue noise characteristics, but the first pixel position matrix (the
matrix for which the threshold values set at the first pixel position are
removed from the dither matrix) and the second pixel position matrix (the
matrix for which the threshold values set at the second pixel position
are removed from the dither matrix) are both matrixes that do not have
blue noise characteristics. Note that as described previously, the first
pixel position and the second pixel position mean pixel positions in a
mutual relationship for which when dots are formed either during forward
scan or backward scan, the other has dots formed by the other.

[0130] Next, the read matrix is set as matrix A (step S202). Then, from
the dither matrix A, two pixel positions (pixel position P and pixel
position Q) are randomly selected (step S204), the threshold value set at
the selected pixel position P and the threshold value set at the selected
pixel position Q are transposed, and the obtained matrix is used as
matrix B (step S206).

[0131] Next, the granularity evaluation value Eva for the matrix A is
calculated (step S208). Here, the granularity evaluation value means an
evaluation value obtained as follows. First, using the dither method on
256 images of tone values 0 to 255, 256 images are obtained expressed by
the presence or absence of dot formation. Next, each image is broken down
into forward scan images and backward scan images. As a result, for each
of the tone values from 0 to 255, obtained are the forward scan image,
the backward scan image, and an image for which these are overlapped
(total image). For the 768 (=256.times.3) images obtained in this way,
after calculation of the granularity index described previously using
FIG. 11, the value obtained by finding the average value of these is used
as the granularity evaluation value. Note that when calculating the
granularity evaluation value, rather than simply using an arithmetic mean
of the 768 granularity indices, it is also possible to take a weighted
average respectively of the forward scan image, the backward scan image,
and the total image. Alternatively, for a specific tone value (e.g. a low
tone range for which it is said that dots stand out relatively easily),
it is also possible to apply a large weighting coefficient and take the
average. At step S208 of FIG. 16, for the matrix A, this kind of
granularity evaluation value is found, and the obtained value is used as
the granularity evaluation value Eva.

[0132] When the granularity evaluation value Eva is obtained for the
matrix A, the granularity evaluation value Evb is calculated in the same
manner for the matrix B as well (step S210). Next, the granularity
evaluation value Eva for the matrix A and the granularity evaluation
value Evb for the matrix B are compared (step S212). Then, when it is
determined that the granularity evaluation value Eva is bigger (step
S212: yes), the matrix B for which the threshold values set in the two
pixel positions are transposed is through to have more desirable
characteristics than the matrix A which is the source. In light of this,
in this case, the matrix B is reread as matrix A (step S214). Meanwhile,
when it is decided that the granularity evaluation value Evb of the
matrix B is larger than the granularity evaluation value Eva of the
matrix A (step S212: no), then matrix is not reread.

[0133] In this way, only in the case when it is determined that the
granularity evaluation value Eva of the matrix A is larger than the
granularity evaluation value Evb of the matrix B, when the operation of
rereading the matrix B as the matrix A, a determination is made of
whether or not the granularity evaluation values are converged (step
S216). Specifically, the dither matrix set as the source has the dots
formed during the forward scan and the dots formed during the backward
scan generated with imbalance, so immediately after starting the kind of
operation noted above, a large value is taken for the granularity
evaluation value. However, by transposing the threshold values set in the
two pixel position locations, when a smaller granularity evaluation value
is obtained, if the matrix for which the threshold value is transposed is
used, and the operation described above is further repeated for this
matrix, the obtained granularity evaluation value becomes smaller, and it
is thought that over time it becomes stable at a certain value. At step
S216, a determination is made of whether or not the granularity
evaluation value has stabilized, or said another way, whether or not it
can be thought of as having reached bottom. For whether or not the
granularity evaluation values have converged, for example, when the
granularity evaluation value Evb of the matrix B is smaller than the
granularity evaluation value Eva of the matrix A, the decrease volume of
the granularity evaluation value is obtained, and if this decrease volume
is a fixed value or less that is stable across a plurality of operations,
it can be determined that the granularity evaluation values have
converged.

[0134] Then, when it is determined that the granularity evaluation values
have not converged (step S216: no), the process backwards to step S204,
and after selecting two new pixel positions, the subsequent series of
operations is repeated. While repeating this kind of operation, over
time, the granularity evaluation values converge, and when it is
determined that the granularity evaluation values have converged (step
S216: yes), the matrix A at that time becomes a dither matrix having the
previously described "first characteristics" and "second
characteristics." In light of this, this matrix A is stored (step S218),
and the dither matrix generating process shown in FIG. 16 ends.

[0135] If tone number conversion processing is performed while referencing
a dither matrix obtained in this way, and a decision is made on the
presence or absence of dot formation for each pixel, it goes without
saying for the overall image, as well as for the forward scan images and
the backward scan images, that it is possible to obtain images for which
the dots are dispersed well. Because of this, for example even when there
is slight displacement of the dot formation positions during
bidirectional printing, it is possible to suppress to a minimum the
effect on the image quality by this.

[0136] Note that with this embodiment, the granularity evaluation value
Eva used to evaluate the dither matrix is calculated based on the
granularity index that is the subjective evaluation value that uses the
visual sensitivity characteristic VTF, but it is also possible to
calculate based on the RMS granularity that is the standard deviation of
the density distribution, for example.

[0137] The granularity index is a well known method and is an evaluation
index used widely from the past. However, calculation of the granularity
index, as described previously, means obtaining the power spectrum FS by
doing Fourier transformation of an image, and it is necessary to add a
weighting to the obtained power spectrum FS that correlates to the human
visual sensitivity characteristics VTF, so there is the problem of the
calculation volume becoming very large. Meanwhile, the RMS granularity is
an objective measure representing variance of dot denseness, and this can
be calculated simply just by the smoothing process using a smoothing
filter set according to the resolution and calculation of the standard
deviation of the dot formation density, so it is perfect for optimization
processing which has many repeated calculations. In addition, use of the
RMS granularity has the advantage of flexible processing being possible
considering the human visual sensitivity and visual environment according
to the design of the smoothing filter in comparison to the fixed process
that uses the human visual sensitivity characteristics VTF.

[0138] Also, with the embodiment described above, the first pixel position
and the second pixel position were described as pixel positions having a
mutual relationship whereby when dots are formed by either of the forward
scan or the backward scan, with the other, dots are formed by the other.
Specifically, even within a row of pixels aligned in the main scan
direction (this kind of pixel alignment is called a "raster"), there are
cases when a first pixel position and a second pixel position are
included. However, from the perspective of securing image quality during
occurrence of dot position misalignment, it is preferable that the first
pixel positions and the second pixel positions not be mixed within the
same raster. Following is a description of the reason for this.

[0139] FIG. 17 is an explanatory drawing showing the reason that it is
possible to ensure image quality when dot position misalignment occurs by
not mixing the first pixel positions and the second pixel positions
within the same raster. The black circles shown in the drawing indicate
dots formed during the forward scan, and the black squares indicate dots
formed during the backward scan. Specifically, if one of the black
circles or black squares is set as the first pixel position, then the
other is set as the second pixel position. FIG. 17 (a) represents a state
in which the first pixel position and the second pixel position are mixed
in the same raster, and FIG. 17 (b) represents a state in which the first
pixel position and the second pixel position are not mixed in the same
raster. Also, in the respective drawings, the drawing shown at the left
side indicates an image in a state without dot position misalignment, and
the drawing at the right side indicates an image in a state with dot
position misalignment. As is clear from FIG. 17 (a), when the first pixel
position and the second pixel positions are mixed in the same raster,
when dot position misalignment occurs, by the distance between dots
within the raster occurring at close locations and at distant locations,
this degrades the image quality. In comparison to this, as shown in FIG.
17 (b), if the first pixel position and the second pixel position are not
mixed in the same raster, for example, even when dot position
misalignment occurs, there is no occurrence of the dot distance in a
raster being at close locations and distant locations, and it is possible
to suppress degradation of the image quality.

[0140] In addition, as shown in FIG. 17 (b), if the first pixel position
rasters and the second pixel position rasters are arranged alternately,
for example, even when dot position misalignment occurs, the dots are
displaced in one direction across the subsequent rasters, and it is
possible to avoid having this visually recognized, degrading the image
quality.

[0141] As described above, the first pixel position dither matrix and the
second pixel position dither matrix are dither matrixes having blue noise
characteristics (or green noise characteristics), and in addition, if the
first pixel positions and the second pixel positions are made not to be
mixed within the same raster, for example even if the dot formation
positions are displaced during bidirectional printing, it is possible to
more effectively suppress this from causing degradation of the image
quality.

F. Variation Examples

[0142] Above, a number of embodiments of the invention were described, but
the invention is in no way limited to these kinds of embodiments, and it
is possible to embody various aspects in a scope that does not stray from
the key points. For example, the following kinds of variation examples
are possible.

[0143] F-1. First Variation Example

[0144] FIG. 18 is an explanatory drawing showing the printing state using
a line printer 200L having a plurality of printing heads 251 and 252 for
the first variation example of the invention. The printing head 251 and
the printing head 252 are respectively arranged in a plurality at the
upstream side and the downstream side. The line printer 200L is a printer
that outputs at high speed by performing only Sub-scan feed without
performing the main scan.

[0145] Shown at the right side of FIG. 18 is a dot pattern 500 formed by
the line printer 200L. The numbers 1 and 2 inside the circles indicate
that it is the printing head 251 or 252 that is in charge of dot
formation. In specific terms, dots for which the numbers inside the
circle are 1 and 2 are respectively formed by the printing head 251 and
the printing head 252.

[0146] Inside the bold line of the dot pattern 500 is an overlap area at
which dots are formed by both the printing head 251 and the printing head
252. The overlap area makes the connection smooth between the printing
head 251 and the printing head 252, and is provided to make the
difference in the dot formation position that occurs at both ends of the
printing heads 251 and 252 not stand out. This is because at both ends of
the printing heads 251 and 252, the individual manufacturing difference
between the printing heads 251 and 252 is big, and the dot formation
position difference also becomes bigger, so there is a demand to make
this not stand out clearly.

[0147] In this kind of case as well, the same phenomenon as when the dot
formation position is displaced between the forward scan and the backward
scan as described above occurs due to the error in the mutual positional
relationship of the printing heads 251 and 252, so it is possible to try
to improve image quality by performing the same process as the embodiment
described previously using the pixel position group formed by the
printing head 251 and the pixel position group formed by the printing
head 252.

[0148] F-2. Second Variation Example

[0149] FIG. 19 is an explanatory drawing showing the state of printing
using the interlace recording method for the second variation example of
the invention. The interlace recording method means a recording method
used when the nozzle pitch k "dots" are 2 or greater measured along the
Sub-scan direction of the printing head. With the interlace recording
method, a raster line that cannot be recorded between adjacent nozzles
with one main scan is left, and the pixels on this raster line are
recorded during another main scan. With this variation example, the main
scan is also called a pass.

[0150] FIG. 19 (A) shows an example of the Sub-scan feed when using four
nozzles, and FIG. 19 (B) shows the parameters of that dot recording
method. In FIG. 19 (A), the solid line circles containing numbers
indicate the Sub-scan direction position of the four nozzles for each
pass. Here, "pass" means one main scan. The numbers 0 to 3 in the circles
mean the nozzle numbers. The position of the four nozzles is sent in the
Sub-scan direction each time one main scan ends.

[0151] As shown at the left end of FIG. 19 (A), with this example, the
Sub-scan feed volume L is a fixed value of four dots. Therefore, each
time a Sub-scan feed is performed, the four nozzle positions are
displaced in the Sub-scan direction four dots at a time. Each nozzle has
as a recording subject all the dot positions (also called "pixel
positions") on the respective raster lines in one main scan. At the right
end of FIG. 19 (A) is shown the number of the nozzle that records the
dots on each raster line.

[0152] In FIG. 19 (B) are shown the various parameters relating to this
dot recording method. Included in the parameters of the dot recording
method are nozzle pitch k [dots], used nozzle count N [units], and
Sub-scan feed volume L [dots]. With the example in FIG. 19, the nozzle
pitch k is three dots. The used nozzle count N is four units.

[0153] Shown in the table in FIG. 19 (B) are the Sub-scan feed volume L
for each pass, the cumulative value .SIGMA.L thereof, and the nozzle
offset F. Here, the offset F is a value that, when a reference position
is assumed for which the offset is 0 for a cyclical position of the
nozzles for the first pass 1 (in FIG. 19, the position at every four
dots), indicates by how many dots the nozzle position for each pass after
that is separated in the Sub-scan direction from the reference position.
For example, as shown in FIG. 19 (A), after pass 1, the nozzle position
moves in the Sub-scan direction by an amount Sub-scan feed volume L (four
dots). Meanwhile, the nozzle pitch k is three dots. Therefore, the offset
F of the nozzles for pass 2 is 1 (see FIG. 19 (A)). Similarly, the nozzle
position for pass 3 is .SIGMA.L=8 dots moved from the initial positions,
and the offset F is 2. The nozzle position for pass 4 is .SIGMA.L=12 dots
moved from the initial position, and the offset F is 0. With pass 4 after
three Sub-scan feeds, the nozzle offset F backwards to 0, so with three
Sub-scans as one cycle, by repeating this cycle, it is possible to record
all the dots on the raster line in an effective recording range.

[0154] In this way, with the second variation example, in contrast to
embedding the dots with the forward scan and backward scan as described
above, dots are embedded with one cycle three passes, so it is
conceivable that there will be displacement of mutual positions between
each pass in one cycle due to Sub-scan feed error. Because of this, it is
possible that the same phenomenon will occur as when the dot formation
positions are displaced with the forward scan and backward scan described
above, so it is possible to try to improve the image quality using the
same process as the embodiments described above with a pixel position
group formed with the first pass of each cycle, a pixel position group
formed with the second pass, and a pixel position group formed with the
third pass.

[0155] Note that with the interlace recording method, each cycle does not
necessarily embed dots with three passes, and it is also possible to
constitute one cycle with two times or four times or more. In this case,
it is possible to do group division for each pass that constitutes each
cycle.

[0156] Also, the group division does not necessarily have to be performed
on all the passes that constitute each cycle, and for example, it is also
possible to constitute this to be divided into a pixel position group
formed with the last pass of each cycle for which Sub-scan feed error
accumulation is anticipated and a pixel position group formed with the
first pass of each cycle.

[0157] F-3. Third Variation Example

[0158] FIG. 20 is an explanatory drawing showing the state of printing
using an overlap recording method for the third variation example of the
invention. In FIG. 20, the solid line circles including numbers indicate
positions in the Sub-scan direction of six nozzles for each pass. The
numbers 1 to 8 in the solid line circles are the number of remainders
after dividing the pass number by 8. The pixel position number indicates
the sequence of the arrangement of pixels on each raster line.

[0159] The overlap recording method is a recording method for which each
raster line is formed by a plurality of passes. With the third variation
example, each raster line is formed with two passes. In specific terms,
for example, the raster line for which the raster number is 1 is formed
by pass 1 and pass 5, and the raster lines 2 and 3 are respectively
formed by pass 8 and pass 4, and pass 3 and pass 7.

[0160] As can be seen from FIG. 20, the dot pattern constituted by the
raster lines for which the raster numbers are 1 to 4 are formed by eight
passes of pass 1 to pass 8, and the dot pattern constituted by the raster
lines for which the raster numbers are 5 to 8 are formed by eight passes
of pass 3 to pass 10. Furthermore, when we focus on the number of
remainders when the pass number is divided by 8, by repeating the dot
pattern constituted by the dots formed on pixels 1 to 4 by the raster
number and pixel position numbers 1 to 4, we can see that all the dot
patterns are formed.

[0161] FIG. 21 is an explanatory drawing showing the eight pixel position
groups divided according to the number of remainders when the pass number
is divided by 8. With FIG. 21, each square shape indicates an image area
constituted by pixels for which the pixel position number is 1 to 4 of
the raster lines for which the raster number is 1 to 4. This image area
correlates to the "shared printing area" in the patent claims, and is
constituted by combining the print pixels belonging to each of the eight
pixel position groups.

[0162] In this kind of case as well, the same phenomenon occurs as when
there is mutual displacement of the dot positions formed with each pass,
so it is possible to attempt to improve the image quality by performing
the same process as the embodiments described above so that the dots
formed by each of the eight pixel position groups has specified
characteristics.

[0163] F-4. Fourth Variation Example

[0164] FIG. 22 is an explanatory drawing showing an example of the actual
printing state for the bidirectional printing method of the third
variation example of the invention. The letters in the circles indicate
which of the forward or backward main scans the dots were formed with.
FIG. 22 (a) shows the dot pattern when displacement does not occur in the
main scan direction. FIG. 22 (b) and FIG. 22 (c) show the dot patterns
when displacement does occur in the main scan direction.

[0165] With FIG. 22 (b), in relation to the position of dots formed at the
print pixels belonging to the pixel position group for which dots are
formed during the forward movement of the printing head, the position of
the dots formed at the print pixels belonging to the pixel position group
for which dots are formed during the backward scan of the printing head
is shifted by 1 dot pitch in the rightward direction. Meanwhile, with
FIG. 22 (c), in relation to the position of the dots formed at the print
pixels belonging to the pixel position group for which dots are formed
during the forward scan of the printing head, the position of the dots
formed at the print pixels belonging to the pixel position group for
which dots are formed during the backward scan of the printing head is
shifted by 1 dot pitch in the leftward direction.

[0166] With the embodiments described above, by giving blue noise or green
noise spatial frequency distribution to both the dot patterns of the
pixel position group for which dots are formed during the forward scan
and the dot patterns of the pixel position group for which dots are
formed during the backward scan, image quality degradation due to this
kind of displacement is suppressed.

[0167] In contrast to this, the third variation example is constituted so
that the dot pattern for which the dot pattern formed on the pixel
position group formed during the forward scan and the dot pattern formed
on the pixel position group formed during the backward scan are shifted
by 1 dot pitch in the main scan direction and synthesized has blue noise
or green noise spatial frequency distribution, or has a small granularity
index.

[0168] The constitution of the dither matrix focusing on the granularity
index can be constituted so that, for example, the average value of the
granularity index when the displacement in the main scan direction is
shifted by 1 dot pitch in one direction, when it is shifted by 1 dot
pitch in the other direction, and when it is not shifted, is a minimum.
Alternatively, it is also possible to constitute this such that the
spatial frequency distributions in these cases have a mutually high
correlation coefficient.

[0169] Note that this variation example is able to increase the robustness
level of the image quality in relation to displacement of the dot
formation position during forward scan and backward scan, so it is
possible to suppress the degradation of image quality not only in cases
when the dot formation positions are shifted as a mass during the forward
scan and the backward scan, but also when unspecified displacement occurs
with part of the pixel position group for which dots are formed during
the forward scan and the pixel position group for which dots are formed
during the backward scan. For example, it is possible to suppress
degradation of the image quality also in cases such as when there is
partial variation in the gap of the printing head and the printing paper
between the forward scan and the backward scan due to cyclical
deformation due to the main scan of the main scan mechanism of the
printing head, for example.

[0170] F-5. This invention can also be applied to printing that performs
printing using a plurality of printing heads. In specific terms, it is
also possible to constitute this so that the spatial frequency
distributions of dots formed in a plurality of pixel position groups in
charge of dot formation by each of the plurality of printing heads have a
mutually high correlation coefficient.

[0171] By working in this way, for printing using the plurality of
printing heads, it is possible to constitute halftone processing with a
high robustness level to displacement of dot formation positions between
mutual printing heads, for example.

[0172] F-6. With this invention, the inventors found not only robustness
in relation to dot formation position misalignment, but also suppression
of degradation of image quality due to the dot formation time sequence
(or dot formation timing displacement).

[0173] FIG. 23 is an explanatory drawing showing the state of print images
being formed by mutually combining in a shared printing area four image
groups in a case when conventional halftone processing is performed. FIG.
23 shows the dot patterns when the four to one pixel position groups are
respectively combined.

[0174] With conventional halftone processing, processing is performed with
a focus on the print image dot dispersion properties formed by all the
pixel position groups, so as can be seen from FIG. 23, there is
unevenness in the dot dispersion properties of each pixel position group.
Specifically, a dense dot state occurs in the low frequency area. This
kind of dense dot state causes a state of accumulation of ink drops,
excessive sheen, and a bronzing phenomenon at the positions where the dot
density is high, and causes image differences with positions at which dot
density is low. This image difference causes the problem of it being easy
for the human visual sense to recognize this as image unevenness.

[0175] This invention suppresses excessive high density of dots and
reduces the states of accumulation of ink drops, excessive sheen, and the
bronzing phenomenon, and causes uniformity for the overall print image,
so it is able to suppress image unevenness. In this way, this invention
is able to be applied broadly to printing that forms print images by
mutually combining in a common print area print pixels belonging to each
of a plurality of pixel position groups, and even if mutual displacement
of dots formed in the plurality of pixel position groups is not assumed,
it can be applied also in cases when there is a difference in timing of
formation of dots formed in the plurality of pixel position groups. This
invention generally can be applied in cases when, for dot formation,
print pixels belonging to each of the plurality of pixel position groups
for which a physical difference is assumed such as displacement of time
or formation position are mutually combined in a common print area to
form a print image.

[0176] F-7. With the embodiments described above, halftone processing was
performed using a dither matrix, but it is also possible to use this
invention in cases when halftone processing is performed using error
diffusion, for example. Using error diffusion can be realized by having
error diffusion processing performed for each of a plurality of pixel
position groups, for example.

[0177] Note that with the dither method of the embodiments noted above, by
comparing for each pixel the threshold values set in the dither matrix
and the tone values of the image data, the presence or absence of dot
formation is decided for each pixel, but it is also possible to decide
the presence or absence of dot formation by comparing the threshold
values and the sum of the tone values with a fixed value, for example.
Furthermore, it is also possible to decide the presence or absence of dot
formation according to the data generated in advance based on threshold
value as and on the tone values without directly using the threshold
values. The dither method of this invention generally can be a method
that decides the presence or absence of dot formation according to the
tone value of each pixel and the threshold value set for the pixel
position corresponding to the dither matrix.

[0178] This invention also includes the following configuration as
examples. With the printing apparatuses described above, it is also
possible to have it so that the halftone process is constituted so that,
at least for the tone level with relatively low dot density, the
correlation coefficient between each of the spatial frequency
distributions of the dot pattern formed on the print pixels belonging to
each of the plurality of pixel position groups and the spatial frequency
distribution of the print image is higher than any of the correlation
coefficients between each of the spatial frequency distributions of dot
patterns formed on print pixels belonging to each of any of the other
plurality of the pixel position groups that form print images by mutually
combining a common print area and the spatial frequency distribution of
the print image, or the halftone process is constituted so that, at least
for the tone level with relatively low dot density, the RMS granularity
of the dot pattern formed on the print pixels belonging to each of the
plurality of pixel position groups is lower than the RMS granularity of
the dot pattern formed on the print pixels belonging to each of any of
the other of the plurality of pixel position groups that form print
images by mutually combining a common print area.

[0179] In this way, with this invention, it is acceptable as long as the
optimality for the plurality of pixel position groups to be evaluated is
compensated.

[0180] This invention further provides a printing apparatus with the
following aspects. Specifically, a printing apparatus that prints images
by forming dots both during forward scan and backward scan of the dot
forming head, comprising: a dither matrix for which a threshold value is
set for each pixel, dot formation presence or absence decision means that
receives image data representing the tone value of each pixel
constituting an image and decides the presence or absence of dot
formation for each pixel according to the tone value of each of the
pixels and to the threshold value set for the pixel position
corresponding to the dither matrix, and dot forming means that forms dots
based on the results of deciding the dot formation presence or absence,
and the dither matrix is a matrix having either blue noise
characteristics or green noise characteristics for both threshold value
distribution set for the first pixel position group used for deciding the
presence or absence of dot formation for the pixels for which dots are
formed with either the forward scan or the backward scan of the dot
forming head, and the threshold value distribution set for the second
pixel position group excluding the first pixel position group from the
dither matrix.

[0181] Also, the printing method of this invention corresponding to the
printing apparatus noted above is a printing method that prints images by
forming dots both during forward scan and backward scan of the dot
forming head, comprising: a first step that stores the dither matrix for
which the threshold values are set for each pixel, a second step that
receives image data representing the tone value of each pixel
constituting an image and decides the presence or absence of dot
formation for each pixel according to the tone value of each of the
pixels and to the threshold value set for the pixel position
corresponding to the dither matrix, and a third step that forms dots
based on the results of deciding the dot formation presence or absence,
and the dither matrix stored at the first step is a matrix having either
blue noise characteristics or green noise characteristics for both
threshold value distribution set for the first pixel position group used
for deciding the presence or absence of dot formation for the pixels for
which dots are formed with either the forward scan or the backward scan
of the dot forming head, and the threshold value distribution set for the
second pixel position group excluding the first pixel position group from
the dither matrix.

[0182] With the printing apparatus and printing method according to the
invention of this application, the presence or absence of dot formation
for each pixel is decided while referring to a dither matrix like the
following. Specifically, each pixel position of the dither matrix can be
classified as either a first pixel position or a second pixel position,
and the matrix is such that the distribution of the threshold values set
for the first pixel position and the distribution of the threshold values
set for the second pixel position all have either blue noise
characteristics or green noise characteristics. Here, the first pixel
position and second pixel position means pixel positions having a
relationship such that when forming dots while moving the dot forming
head back and fort, with one of the pixel positions, dots are formed with
either the forward scan or the backward scan, and with the other pixel
position, dots are formed with the other. Note that just because it is
said that each pixel position on the dither matrix can be classified as a
first pixel or a second pixel position, doesn't necessarily mean that the
direction for forming the dot of each pixel position is fixed to the
forward scan or the backward scan.

[0183] Also, the distribution of the threshold values having blue noise
characteristics means the following kind of distribution. Specifically,
when dots are generated using a dither matrix having that kind of
threshold value distribution, dots are generated irregularly, and the
spatial frequency component of the set threshold value means the
distribution of a threshold value such as one having the biggest
component in the high frequency range with one cycle as two pixels or
less. Also, distribution of threshold values having green noise
characteristics are distributions like the following. Specifically, when
dots are generated using a dither matrix having that kind of threshold
value distribution, dots are generated irregularly, and the spatial
frequency component of the set threshold value means the distribution of
a threshold value such as one having the biggest component in the medium
frequency range with one cycle as from two pixels to ten or more pixels.

[0184] The detailed principle is described in detail later, but the
degradation of image quality that occurs when the dot formation position
is displaced between forward scan and backward scan when doing
bidirectional printing can be greatly suppressed by suitably dispersing
dots for both images made only with dots formed during the forward scan
and images made only by dots formed during the backward scan. As is well
known, using a dither matrix having blue noise characteristics or green
noise characteristics, it is possible to suitably disperse dots if the
presence or absence of dot formation is decided for each pixel.
Therefore, if a dither matrix such as one having respectively blue noise
characteristics or green noise characteristics is used for the
distribution of threshold values set for the first pixel positions and
distribution of threshold values set for the second pixel positions, it
is possible to suitably disperse dots for both images made only with dots
formed during the forward scan and images made only by dots formed during
the backward scan, and thus, it is possible to suppress to a minimum the
degradation of image quality when there is displacement of the dot
formation positions.

[0185] Also, with this kind of printing apparatus, it is also possible to
decide the presence or absence of dot formation for each pixel while
referencing the following kind of dither matrix. Specifically, it is also
possible to reference a dither matrix such as one for which, when the
pixel positions of the matrix are classified into rasters that are pixel
positions aligned in the direction in which the dot formation head moves
back and forth, only one or the other of the first pixel position or the
second pixel position is contained within one of the rasters.

[0186] By working in this way, even if there is dot formation position
misalignment between the dot formation head forward scan and backward
scan, within the same raster, dots are formed only of one or the other of
the forward scan or backward scan, and the distance between dots does not
come too close or too far, so it is possible to suppress degradation of
the image quality.

[0187] Also, with this kind of dither matrix, it is also possible to align
rasters containing only first pixel positions and rasters containing only
second pixel positions alternately in a direction intersecting with the
raster.

[0188] By working in this way, with dots formed during forward scan and
dots formed during backward scan, even if the dot formation positions are
displaced, the dots are displaced in one direction over consecutive
rasters, and it is possible to avoid this from being visible and
degrading the image quality.

[0189] Also, the printing apparatus described above forms dots based on
the presence or absence of dot formation decided for each pixel, and when
the presence or absence of dot formation for each pixel is decided, if
the focus is on deciding this by referencing a dither matrix having
specified characteristics, the invention of this application can also be
understood as the following kind of image processing device and image
processing method. Specifically, the image processing device of the
invention of this application is an image processing device that
generates control data used for a printing apparatus that prints images
by forming dots both during forward scan and backward scan of the dot
forming head to control the dot formation, comprising: a dither matrix
for which threshold values are set for each pixel, dot formation presence
or absence decision means that receives image data representing the tone
value of each pixel constituting an image and decides the presence or
absence of dot formation for each pixel according to the tone value of
each of the pixels and to the threshold value set for the pixel position
corresponding to the dither matrix, and control data output means that
outputs the results of deciding the dot formation presence or absence as
the control data, and the dither matrix is a matrix having either blue
noise characteristics or green noise characteristics for both threshold
value distribution set for the first pixel position group used for
deciding the presence or absence of dot formation for the pixels for
which dots are formed with either the forward scan or the backward scan
of the dot forming head, and the threshold value distribution set for the
second pixel position group excluding the first pixel position group from
the dither matrix.

[0190] The method of generating this kind of dither matrix is a method of
generating a dither matrix for printing that forms a print image by
mutually combining in a common print area the print pixels belonging to
each of the plurality of pixel position groups for which physical
differences are assumed during dot formation, comprising: setting of the
evaluation function that sets the evaluation function for calculating the
evaluation value of the dither matrix, preparing that prepares a dither
matrix as the initial state for storing in each element a plurality of
threshold values for deciding the presence or absence of dot formation
for each pixel according to the input tone value, deciding of the storage
elements that, while replacing part of the plurality of threshold values
stored in each element with threshold values stored in other elements,
decides the elements in which each threshold value is stored, and
outputting of a dither matrix for which the storage element is decided
for all of the plurality of threshold values, the deciding of the storage
element including: mutual replacing of part of the plurality of threshold
values, calculating of the evaluation value of the dither matrix for
which the threshold value was replaced using the evaluation function, and
deciding of the storage element for the plurality of threshold values
according to conformity to a specified criterion of the evaluation value,
and the evaluation function is constituted based on the characteristics
of the dot pattern formed on the print pixels belonging to each of the
plurality of pixel position groups at least for the tone level with
relatively low dot density.

[0191] For the dither matrix generating method noted above, it is
preferable that the evaluation function be set to be the RMS granularity
of the dot pattern formed on the print pixels belonging to each of the
plurality of pixel position groups for at least the part of the
gradations for which the dot density is relatively low.

[0192] The RMS granularity is an objective measure representing variation
in the dot denseness, and it is capable of doing simple calculation
simply with a smoothing process using a smoothing filter set according to
the resolution and with calculation of the standard deviation of the dot
formation density, so it is very suitable for optimization processing
which involves many calculation repetitions. In addition, use of RMS
granularity is because it has the advantage of it being possible to do
flexible processing considering human visual sense and visual environment
according to the design of the smoothing filter in comparison to a fixed
process using the human visual sense characteristic VTF.

[0193] This kind of dither matrix generating method is a method of
generating a dither matrix for printing that forms a print image by
mutually combining in a common print area the print pixels belonging to
each of the plurality of pixel position groups for which physical
differences are assumed during dot formation, comprising: setting of the
evaluation function that sets the evaluation function for calculating the
evaluation value of the dither matrix, preparing that prepares a dither
matrix as the initial state for storing in each element a plurality of
threshold values for deciding the presence or absence of dot formation
for each pixel according to the input tone value, deciding of the storage
elements that, while replacing part of the plurality of threshold values
stored in each element with threshold values stored in other elements,
decides the elements in which each threshold value is stored, and
outputting of a dither matrix for which the storage element is decided
for all of the plurality of threshold values, the deciding of the storage
element including: mutual replacing of part of the plurality of threshold
values, calculating of the overall evaluation value that is the
evaluation value of the dither matrix for which the threshold value was
replaced using the evaluation function, and deciding of the storage
element for the plurality of threshold values according to conformity to
a specified criterion of the overall evaluation value and each of the
group evaluation values.

[0194] Also, the image processing method of this invention corresponding
to the image processing device noted above is an image processing method
that generates control data used for a printing apparatus that prints
images by forming dots both during forward scan and backward scan of the
dot forming head to control the dot formation, comprising: step (A) that
stores the dither matrix for which threshold values are set for each
pixel, step (B) that receives image data representing the tone value of
each pixel constituting an image and decides the presence or absence of
dot formation for each pixel according to the tone value of each of the
pixels and to the threshold value set for the pixel position
corresponding to the dither matrix, and step (C) that outputs the results
of deciding the dot formation presence or absence as the control data,
and the dither matrix stored at the step (A) is a matrix having either
blue noise characteristics or green noise characteristics for both
threshold value distribution set for the first pixel position group used
for deciding the presence or absence of dot formation for the pixels for
which dots are formed with either the forward scan or the backward scan
of the dot forming head, and the threshold value distribution set for the
second pixel position group excluding the first pixel position group from
the dither matrix.

[0195] For this image processing device and image processing method as
well, the same as with the previously describe printing apparatus and
printing method, when deciding the presence or absence of dot formation
for each pixel, the following kind of dither matrix is referenced.
Specifically, referenced is a matrix such as one for which each of the
pixel positions of the dither matrix can be classified as either a first
pixel position or a second pixel position, and both the distribution of
the threshold values set for the first pixel position and the
distribution of the threshold values set for the second pixel position
have either blue noise characteristics or green noise characteristics.
When an image is printed using control data generated referencing this
kind of dither matrix, even if the dot formation positions are displaced
between the dot formation head forward scan and backward scan, it is
possible to suppress to a minimum the degradation of the image quality
due to that, and to print a high image quality image.

[0196] Furthermore, the invention of this application can also be realized
using a computer by reading into a computer a program for realizing the
printing method or the image processing method described above.
Therefore, this invention also includes aspects as the following kind of
program or as a recording medium on which is recorded the program.
Specifically, the program of the invention of this application
corresponding to the printing method described above is a program that
realizes using a computer a method that prints images by forming dots
both during forward scan and backward scan of the dot forming head,
realizing using the computer: a first function that stores the dither
matrix for which threshold values are set for each pixel, a second
function that receives image data representing the tone value of each
pixel constituting an image and decides the presence or absence of dot
formation for each pixel according to the tone value of each of the
pixels and to the threshold value set for the pixel position
corresponding to the dither matrix, and a third function that forms dots
based on the results of deciding the dot formation presence or absence,
and the dither matrix stored by the first function is a matrix having
either blue noise characteristics or green noise characteristics for both
threshold value distribution set for the first pixel position group used
for deciding the presence or absence of dot formation for the pixels for
which dots are formed with either the forward scan or the backward scan
of the dot forming head, and the threshold value distribution set for the
second pixel position group excluding the first pixel position group from
the dither matrix.

[0197] Also, the recording medium of the invention of this application
corresponding to the program noted above is a recording medium on which
is recorded a computer readable program that prints images by forming
dots both during forward scan and backward scan of the dot formation
head, recording functions realized using the computer of: a first
function that stores the dither matrix for which threshold values are set
for each pixel, a second function that receives image data representing
the tone value of each pixel constituting an image and decides the
presence or absence of dot formation for each pixel according to the tone
value of each of the pixels and to the threshold value set for the pixel
position corresponding to the dither matrix, and a third function that
forms dots based on the results of deciding the dot formation presence or
absence, and the dither matrix stored by the first function is a matrix
having either blue noise characteristics or green noise characteristics
for both threshold value distribution set for the first pixel position
group used for deciding the presence or absence of dot formation for the
pixels for which dots are formed with either the forward scan or the
backward scan of the dot forming head, and the threshold value
distribution set for the second pixel position group excluding the first
pixel position group from the dither matrix.

[0198] Also, the program of the invention of this application
corresponding to the image processing method described above is a program
that realizes using a computer a method that generates control data used
for a printing apparatus that prints images by forming dots both during
forward scan and backward scan of the dot forming head to control the dot
formation, realizing using the computer: function (A) that stores the
dither matrix for which threshold values are set for each pixel, function
(B) that receives image data representing the tone value of each pixel
constituting an image and decides the presence or absence of dot
formation for each pixel according to the tone value of each of the
pixels and to the threshold value set for the pixel position
corresponding to the dither matrix, and function (C) that outputs the
results of deciding the dot formation presence or absence as the control
data, and the dither matrix stored by the function (A) is a matrix having
either blue noise characteristics or green noise characteristics for both
threshold value distribution set for the first pixel position group used
for deciding the presence or absence of dot formation for the pixels for
which dots are formed with either the forward scan or the backward scan
of the dot forming head, and the threshold value distribution set for the
second pixel position group excluding the first pixel position group from
the dither matrix.

[0199] Also, the recording medium of the invention of this application
corresponding to the program noted above is a recording medium on which
is recorded to be readable on the computer a program that generates
control data used for a printing apparatus that prints images by forming
dots both during forward scan and backward scan of the dot forming head
to control the dot formation, recording a program realized using the
computer of: function (A) that stores the dither matrix for which
threshold values are set for each pixel, function (B) that receives image
data representing the tone value of each pixel constituting an image and
decides the presence or absence of dot formation for each pixel according
to the tone value of each of the pixels and to the threshold value set
for the pixel position corresponding to the dither matrix, and function
(C) that outputs the results of deciding the dot formation presence or
absence as the control data, and the dither matrix stored by the function
(A) is a matrix having either blue noise characteristics or green noise
characteristics for both threshold value distribution set for the first
pixel position group used for deciding the presence or absence of dot
formation for the pixels for which dots are formed with either the
forward scan or the backward scan of the dot forming head, and the
threshold value distribution set for the second pixel position group
excluding the first pixel position group from the dither matrix.

[0200] If this kind of program or program recorded on a recording medium
is read into a computer and the various functions described above are
realized using the computer, even when the dot formation positions are
displaced between the dot formation head forward scan and backward scan,
it is possible to suppress to a minimum the effect due to this. Because
of this, it is possible to rapidly print high image quality images and
also possible to simplify the mechanism and control for adjusting the dot
formation position with the forward scan and backward scan.

[0201] Finally, the parent of the present application claims priority
based on Japanese Patent Application No. 2005-032771 filed on Feb. 9,
2005, Japanese Patent Application No. 2005-171290 filed on Jun. 10, 2005,
and Japanese Patent Application No. 2005-210792 filed on Jul. 21, 2005,
the disclosures of which are herein incorporated by reference in their
entirety for all purposes.